comprehensive inorganic chemistry ii || oxides: their properties and uses

Co

4.03 Oxides: Their Properties and UsesSPN Nair and P Murugavel, Indian Institute of Technology Madras, Chennai, India

ã 2013 Elsevier Ltd. All rights reserved.

4.03.1 Introduction 474.03.2 Perovskites 474.03.3 Aurivillius Phase 524.03.4 Ruddlesden–Popper Structures 564.03.5 Brownmillerites 584.03.6 Pyrochlores 624.03.7 Spinels 674.03.8 Delafossites 684.03.9 Conclusion 69References 69

4.03.1 Introduction

Oxides are a wide class of materials with different crystal struc-

tures, displaying amazingly diverse properties. These properties

make them quite useful in a large range of applications. This

chapter is an attempt to understand the development in the field

of oxides in the past decade. Some of the important properties of

interest are their dielectric, ferroelectric, piezoelectric, magnetic,

dilute magnetic semiconductor properties, ionic conductivity,

fuel cell characteristics, and optical properties among others. It is

a difficult task to put together all the oxides in one review article.

In this chapter, we begin our discussion on the recent develop-

ments on the perovskite oxide materials. Heterostructure mate-

rials based on these perovskite oxides have surprisingly thrown

up a range of exotic properties. The rest of the chapter focuses on

the crystal structureswith a close relation to perovskite structures.

These include Aurivillius phases, Ruddlesden–Popper (RP)

phases, pyrochlores, brownmillerites, etc. Some of the results

described here are of fundamental interest, such as the field of

spin ice and the observation of magnetic monopoles in spin ice

structures. The Aurivillius phase on the other hand has found

importance in the field of nonvolatile random access memory

devices because of their stability and high ferroelectric Curie

temperature. RP and brownmilleritematerials are useful because

of their complex structural andmagnetic phenomenon aswell as

their oxide ion conduction behavior that can be used in the field

of solid oxide fuel cells (SOFCs). Finally, we discuss the recent

development in the field of spinels and delafossite structures.

4.03.2 Perovskites

In 1839, the mineral CaTiO3 was found in the Ural Mountains

and named perovskite by Gustav Rose in the memory of Count

Lev Aleksevich von Perovskite, a Russian geologist.1 The perov-

skite has the stoichiometric ABO3 formula, where ‘A’ and ‘B’ are

cations and ‘O’ is an anion (Figure 1). Both the cations can take a

variety of charges. Like in the original mineral, CaTiO3, the A

cation is divalent and the B cation is tetravalent, which results in

an orthorhombic structure with the space group Pnma.2,3 The

radii of the ions in an ideal cubic perovskite constructed with the

mprehensive Inorganic Chemistry II http://dx.doi.org/10.1016/B978-0-08-09777

cation sphere in contact with an oxygen anion sphere can be

estimated by RA þ RO ¼ ffiffiffi2

pRB þ ROð Þ where RA, RB, and RO are

the relative ionic radii of the A-site and B-site cations and

the oxygen ion, respectively. However, with decreasing A cation

size, the cation will not be in a position to be in contact with the

anions. To bring the cation in contact with the anion, the B–O–B

links bend slightly resulting in tilting of the BO6 octahedra.4 The

resultant degree of distortion of a perovskite from ideal cubic

structure is given by the tolerance factor t. The factor t is related

to the ionic radii by the equation, RA þ RO ¼ tffiffiffi2

pRB þ ROð Þ.5

The limiting values for the tolerance factor have been deter-

mined through experiments. Although early studies showed

mainly cubic or pseudocubic structure for perovskite, later stud-

ies suggest that many of the materials exhibit the orthorhombic

Pnma distorted structure at room temperature (RT).6,7 As the

distortion increases, the perovskite goes to a rhombohedral

structure with the space group R3c8 and a hexagonal P63cm

structure with further increase in distortion.9 Based on the anal-

ysis of tolerance factor, it is understood that the perovskite will

be orthorhombic if 0.75< t<0.9, cubic if 0.9< t<1.0, and hex-

agonal if 1.00< t<1.13.

Normally, the properties of the materials are closely associ-

ated with its structure, but this is not the case for the perovskites.

Theperovskite structure, uponclever chemicalmanipulation, can

lead to a wide variety of phases with totally different functions.

Materials with the perovskite structure display properties from

insulators to superconductors, piezoelectrics to relaxor ferroelec-

trics, and magnetoresistance to magnetoelectrics (MEs). In this

chapter, the focus is on the more recent development related to

multiferroics and perovskite-based superlattice structures.

Multiferroics are interesting because they exhibit simulta-

neous ferromagnetic and ferroelectric polarizations and the

coupling between them. The magnetic polarization can be

switched by applying an electric field and the ferroelectric

polarization can be switched by applying a magnetic field,

which gives ample opportunities for designing novel devices.

However, single-phase materials in which ferroelectricity and

ferromagnetism arise independently are rare in nature. As an

alternative, multiferroics composites comprising of both ferro-

electric and ferromagnetic phases also yield ME coupling. It

is generated as a product property of a magnetostrictive and

4-4.00403-4 47

http://dx.doi.org/10.1016/B978-0-08-097774-4.00403-4

(a) (b)

(d)

c

b

b

c

a

(c)

b ac

a

Figure 1 (a) Cubic perovskite unit cell, (b) orthorhombic perovskite unit cell, (c) rhombohedral unit cell, and (d) hexagonal perovskite unit cell, whereblue, yellow, and red spheres represent A cation, B cation, and oxygen anions, respectively.

(a) (b)

Figure 2 (a) Unit cell view: O2 – large sphere, Bi3þ – medium sphere,and Mn3þ – small sphere. (b) MnO6 octahedra and bismuth cations in theperovskite unit. Reproduced from Atou, T.; Chiba, H.; Ohoyama, K.;Yamaguchi, Y.; Syono, Y. J. Solid State Chem. 1999, 145, 639–642, withpermission from Elsevier.

48 Oxides: Their Properties and Uses

piezoelectric material. Some of the single-phase complex perov-

skite materials exhibit these multiple functional properties and

they have gained importance in the recent past. In addition,

perovskite-based multiferroics composites have gained impor-

tance because of their large ME coupling. Furthermore, new

multiferroics are made in the form of thin films using ferroelec-

tric and magnetic compounds to fabricate nanocomposites, or

superlattice, or heterostructures.

Most of the single-phase multiferroics can be either Bi-based

perovskites or rare-earth-based perovskites, both with high fer-

roelectric Curie temperature. However, the magnetic transition

varies from low temperature to RT depending on the structure of

the materials. Bi-based compounds are BiMnO3 and BiFeO3.

Recently, several attempts have been made to enhance the ME

coupling in these oxides through Bi site substitutions.

BiMnO3: Bismuth manganite shows ferroelectric transition

at 750 K and ferromagnetic transition at 105 K.10 The neutron

and electron diffraction (ED) studies on the stable BiMnO3

synthesized by a high-pressure technique reveal a distorted pe-

rovskite structure11,12 as shown in Figure 2. It is in monoclinic

symmetry with C2 as the space group. The lattice parameters are

a¼9.5323 A, b¼5.6064 A, c¼9.8535 A, and b¼110.667.11,12

The distortion is caused by the polarized Bi 6s2 lone

pair, which is predicted to be the origin of ferroelectricity

in this compound by the first principles electronic structure

calculations.13 The lattice instability to the off-centered displace-

ment is due to the strong covalent bonding between Bi 6p andO

2p states.14 The ferromagnetism is originated from the orbital

ordering ofMn 3d states. The Jahn–Teller distortion of theMn3þ

cation and the ordering of a vacant dx2�y2 orbital are suggested

to play an important role in the origin of ferromagnetism.11 The

magnetocapacitance studies revealed a large effect near its ferro-

magnetic transition indicating the existence of the coupling

between the ferromagnetic and the ferroelectric ordering.15

The properties of BiMnO3 are slightly altered when it was

made in thin film form. Various techniques were used for thin

film fabrication.10,16–19 The pulsed laser deposition of BiMnO3

on single crystalline (100)LaAlO317 and (111)SrTiO3

18 substrates

reveal monoclinic twinned crystallites oriented in two directions.

The magnetic measurements showed a lower ferromagnetic tran-

sition compared to the bulk compound. Figure 3 shows the

temperature-dependent magnetization of the epitaxial (100)

BiMnO3 film deposited on a LaAlO3 substrate and

the preferentially (111)-oriented BiMnO3 film (100 nm)

Oxides: Their Properties and Uses 49

deposited on a (111)Pt/TiO2/SiO2/Si substrate. Although the

remnant magnetic moment of the epitaxial film is larger than

the polycrystalline film, both of them show a lower Curie tem-

perature of around45 K. Thedecrease inCurie temperature canbe

attributed to various factors such as the nonstoichiometric com-

position, strain, or size effect.19 The application of the multifer-

roics BiMnO3 film on a (111)Pt/TiO2/SiO2/Si substrate in a data

storage device is demonstrated bywriting and reading of nanosize

bits of ferroelectric polarization using Kelvin force microscopy.17

BiFeO3: It is a commensurate ferroelectric20 and an incom-

mensurate antiferromagnet21 at RT, with 1103 K as the ferroelec-

tric Curie temperature22 and 643 K as the antiferromagnetic Neel

temperature.23 In bulk, BiFeO3 crystallizes in the rhombohedral

structure where the unit cell can be visualized as two distorted

perovskite unit cells connected along their body diagonal

denoted as the pseudocubic h111i.24 The compound shows G-

type antiferromagnetic ordering where the Femagnetic moments

are aligned ferromagnetically within pseudocubic (111) planes

and antiferromagnetically between adjacent (111) planes.25 Its

0 150100

BiMnO3/LaAIO3

BiMnO3/Pt/TiO2/SiO2/Si

50−0.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0M/H

(arb

. uni

t.)

0.5

0.0

Figure 3 Temperature dependence of the magnetization of the epitaxial(100) BiMnO3 film and the preferentially (111)-oriented BiMnO3 film(100 nm). Reproduced from Son, J. Y.; Kim, B. G.; Kim, C. H.; Cho, J. H.Appl. Phys. Lett. 2004, 84, 4971–4973, with permission from AIP.

−60

−60

−40

−20

0

20

40

60

−40 −20 0Electric field (MV m-1)

BiFeO3/(100)-SrTiO3

(a) (b

Pol

ariz

atio

n (μ

Ccm

-2)

20 40 60

Figure 4 Ferroelectric hysteresis loop for BiFeO3 measured at a frequency oReproduced from Wang, J. B.; Neaton, H.; Zheng, V.; Nagarajan Ogale, S. B.;Association for the Advancement of Science.

weak ferromagnetism at RT is due to a residual moment from the

canted spin structure. The ferroelectric polarization in this com-

pound is realized by a large displacement of the Bi ions relative

to the FeO6 octahedra with polarization direction along four

cubic diagonals.26 However, the polarization measurement is

hampered by the large leakage currents due to impurity phases,

Fe2þ formation, and oxygen deficiency. Single-phase BiFeO3

ceramics can be prepared by leaching the impurity phases by

dilute nitric acid27 and also by a rapid liquid phase sintering

technique.28 In its pure form, BiFeO3 shows 8.9, 4.0 mC cm�2,

and 39 kV cm�1 as its spontaneous polarization, remnant polar-

ization, and coercive field, respectively.

Interestingly, the structure and properties of BiFeO3 in thin

film form are found to be sensitive to substrate-induced strain29–

31 and growth pressure.32 The films deposited on different sub-

strates revealed different structures. The BiFeO3 films made on

(100)- or (101)-oriented SrTiO3 substrates are in monoclinic

symmetry with 3.935 and 4.0 A as the in-plane and the out-of-

plane parameters, respectively.29 However, it shows a tetragonal

structureonSi substrates.32Thebulk-like rhombohedral structure

is obtained only when the films are grown on the (111) SrTiO3

substrate.31 Interestingly, the filmmade on a SrTiO3 substrate by

a pulsed laser deposition technique showed RT spontaneous

polarization (50–60 mC cm�2) as shown in Figure 4, an order

of magnitude higher than that of the bulk (6.1 mC cm�2).33

However, the remnant polarization of the films grown on

(100)- and (111)-oriented SrTiO3 substrates showed a large

difference indicating the strong influence of substrate-induced

strain on the ferroelectric properties of the BiFeO3 film.34 The

first principle calculations also indicate the sensitivity of the

polarization to small changes in lattice parameters.33,35

To explore the possibility of altering the ferroelectric, mag-

netic, and ME properties of BiFeO3 film by heteroepitaxial

constraints, epitaxial BiFeO3 films have been prepared by

employing various deposition techniques such as pulsed laser

deposition,36,37 radio-frequency sputtering,38,39 metal–organic

chemical vapor deposition,40,41 and chemical solution deposi-

tion on a variety of substrates such as SrTiO3, DyScO3, and

LaAlO3. The electrical properties have been studied by growing

)

BiFeO3/(100)-Si

−60

−30

0

30

60

Pol

ariz

atio

n (μ

Ccm

-2)

−15 −10 −5 0Applied voltage (V)

5 10 15

f 15 kHz on (a) (100) SrTiO3 substrate and (b) (100) Si substrates.et al. Science 2003, 299, 1719, with permission from the American


the films on conducting oxides including SrRuO3, (La, Sr)

MnO3, and LaNiO3 as bottom electrodes. As an example of

the high quality of the grown films, the x-ray diffraction results

of BiFeO3 film grown on (001) SrTiO3 substrate are shown in

Figure 5. The off-axis azimuthal f scan shown in Figure 5(b)

reveals 210R rhombohedral BiFeO3 reflections.39

The periodic domain structure of multiferroics is more

suitable for photonic device applications. In epitaxial BiFeO3

film, a one-dimensional (1D) periodic ferroelectric domain

structure was created by carefully fabricating it on SrRuO3 as

the bottom electrode on a closely lattice-matched (110)

DyScO3 substrate, which is shown in Figure 6.42

One of the major problems in fabricating the pure BiFeO3

film is its high leakage current. It is a big hindrance for fer-

roelectric measurements; as a result, many researchers reported

15100

102

104

106

108

1010

5550454035

(002

) BiF

eO3

600 nm

2q (�)(a)

Inte

nsity

(cp

s)

60 nm

302520

(001

) BiF

eO3

(001

) SrT

iO3

(002

) SrT

iO3

Figure 5 x-Ray diffraction showing (a) wide y–2y scan of 60 and 600 nm fscan of 210R rhombohedral BiFeO3 reflections. Reproduced from Das, R. R.;2006, 88, 242904–242906, with permission from AIP.

[010]c

[100]c

[001]O

[1–10]O

BiFeO3

a1= 3.965 Åa2= 3.965 Å

a2= 3.923 Å

SrRuO3

1m

a1= 3.922 Å

a2= 3.946 Å

a2a1

(a) (b)

DyScO3a1= 3.951 Å

Figure 6 (a) Schematic of the BFO/SRO/DSO heterostructure and (b) polarizatReproduced from Chu, Y. H.; Zhan, Q.; Martin, L. W.; Cruz, M. P.; Yang, P. L.;

varied nature of ferroelectric properties in BiFeO3 films.36,43 Var-

ious possible mechanisms to prevent the leakage current in

ferroelectric perovskite oxides have been discussed.44,45 Among

them, A- or B-site doping is found to considerably lower the

leakage current in BiFeO3 oxides.43 All BiFeO3 films showed a

well-saturated weak ferromagnetic hysteresis loops at RT in agree-

ment with predictions.46

In rare-earth manganites (ReMnO3), as we decrease the

ionic radii, the perovskite structure changes from an ortho-

rhombic to a hexagonal structure. Rare-earth perovskite

manganite compounds with Re¼La, Ce, Nd, . . ., Gd, Tb, Dy

exhibit an orthorhombic structure with Pnma as the space

group and compounds with Re¼Ho, Er, Tm, Yb, Lu, Sc exhibit

hexagonal crystallographic structures with P63cm as the space

group. Interestingly, all the hexagonal perovskite rare-earth

1000 36027018090

101

102

210R BiFeO3

f (�)(b)

Inte

nsity

(cp

s)

ilms of BiFeO3 film on (001) SrTiO3 substrate. (b) Off-axis azimuthal fKim, D. M.; Baek, S. H.; Zavaliche, F.; Yang, S. Y.; et al. Appl. Phys. Lett.

m

ion force microscopic images of BFO films with periodic domain structure.et al. Adv. Mater. 2006, 18, 2307–2311, with permission from Wiley-VCH.

Paraelectric

(a) (b)

Ferroelectric

YMnO3

Figure 8 The crystal structure of YMnO3 in the paraelectric andferroelectric phases. The trigonal bipyramids depict MnO5 polyhedra andthe spheres represent Y ions. (a) The stacking of two consecutive MnO5

layers and the sandwiched Y layer, looking down the c-axis in theparaelectric phase. (b) A view of the ferroelectric phase from theperpendicular to the c-axis, showing the layered nature of YMnO3.Reproduced from Van Aken, B. B.; Palstra, T. T. M.; Filipetti, A.; Spaldin,N. A. Nat. Mater. 2004, 3, 164–170, with permission from NaturePublishing Group.

9 K

E||c

12 K15 K18 K5/e

c(0)

(%)

10


manganites exhibit multiferroic properties in comparison to

the orthorhombic phase where only TbMnO3 and DyMnO3

show multiferroic properties in bulk. As the A-site cationic

radius decreases, the orthorhombic and Jahn–Teller distorted

perovskite structure become less stable and near the Dy/Ho

boundary, the structure becomes hexagonal as shown in

Figure 7. Near the phase boundary, compounds with ortho-

rhombic structure can be stabilized into the hexagonal phase

and vice versa by a special synthesis route.

As an example, DyMnO3 can be stabilized both in the

hexagonal as well as in the orthorhombic structure.48 The

ferroelectric ordering for most of the hexagonal manganites

takes place at 900 K and the magnetic antiferromagnetic order-

ing occurs at 100 K.49 The origin of ferroelectricity in hexago-

nal manganites is due to the noncentrosymmetric nature of its

space group where the polar direction is predominantly along

the c-axis. The hexagonal YMnO3 compound, though Y is not a

rare-earth element, shows structure and properties similar to

rare-earth hexagonal compounds. It crystallizes in the hexago-

nal P63cm space group with lattice parameters a¼6.125 A and

c¼11.41 A at 290 K. The crystal structure of YMnO3 is shown

in Figure 8.50,51 It has 2D distorted MnO5 bipyramids sepa-

rated by distorted layers of rare earths, which results in Mn3þ–O2–Mn3þ frustration along the in-plane exchange paths and

Mn3þ–O2–O2–Mn3þ frustration along interplane exchange

paths.51,52 The compound exhibits ferroelectricity as well as

antiferromagnetic properties with a strong coupling between

them.54 The ferroelectricity in YMnO3 is due to the unusual Y

site coordination and the triangular and layered MnO5 net-

work as shown in Figure 8.

On the other hand, among the orthorhombic manganites,

TbMnO3 and DyMnO3, which crystallize in Pnma, are found to

show multiferroic properties.55 As we decrease the tempera-

ture, below 50 K these compounds show sinusoidal antiferro-

magnetic ordering due to spin frustrations followed by the

emergence of a spontaneous polarization due to magnetoelas-

tically induced lattice modulation at 29 K. These compounds

generally showed gigantic ME and magnetocapacitance effects

0.855.4

6.2

6.0

5.8

5.6

0.90 0.95

REMnO3

ScIn

LuYb Er

YHo

Dy

TbNd

OrthorhombicPnma

HexagonalP63cm

Pr

La

1.00 1.05

Ionic radius of RE (Å)

a (Å

)

1.10 1.15 1.20

Figure 7 Evolution of the lattice structure in REMnO3 as a function ofthe size of the rare earth (RE). Reproduced from Prellier, W.; Singh M. P.;Murugavel, P. J. Phys. Condens. Matter. 2005, 17, R803–R832, withpermission from IOP.

as a consequence of the electric polarization induced by mag-

netic fields as shown in Figure 9.56

Most of the multiferroic rare-earth manganites can be sta-

bilized into both the orthorhombic and the hexagonal phase

in the form of a thin film deposited on an appropriately chosen

substrate having suitable in-plane crystalline symmetry. For

example, hexagonal YMnO3 films can be formed on substrates

which have hexagonal in-plane symmetry as in (111) MgO,

0

0 2 4Magnetic field (T)

6 8

1

2

3

4

0(a)

(b)

9 K

E||a

12 K15 K18 K

Δec(

B)

Δea(

B)/e a

(0) (

%)

Figure 9 Magnetocapacitance and magnetoelectric effects in TbMnO3.Magnetic field-induced change in the dielectric constant ((a) and (b)).Reproduced from Kimura, T.; Goto, T.; Thizaka, K.; Arima, T.; Tokura, Y.Nature 2003, 426, 55–58, with permission from Nature Publishing Group.


(0001) ZnO:Al/(0001) sapphire, (111) Pt/(111) MgO, or

(111) Y-stabilized ZrO2.57,58

On the other hand, orthorhombic YMnO3 films can be

formed on substrates such as (010) SrTiO3106 or (101)

NdGaO3, which has an in-plane cubic symmetry.59 Hence, by

using the effect of substrate-induced strain, one can stabilize

some of the metastable phases in thin film form. This method

can be employed to fabricate newmultiferroic phases either from

existingmultiferroicmaterials or fromnonmultiferroicmaterials.

For example, GdMnO3, which is not amultiferroic compound in

its stable orthorhombic phase, is made into a multiferroic com-

pound by stabilizing it in thin film form in a metastable hexago-

nal phase on (111)Pt/(111)MgO substrate.60 A similarmethod is

adapted to convert some of the stable multiferroic orthorhombic

rare-earth manganites into multiferroic hexagonal manganites

and vice versa on appropriate substrates.61–63

Multiferroic materials are proposed to have potential appli-

cation in various fields such as information storage, spintronics,

four-state memories, etc. However, their application capabilities

depend on their high coupling strength between various order

parameters, which is found to be very low inmost of the known

single-phase multiferroic materials reported so far. However, the

large ME response observed in multiferroics ME composites,64

which are made by combining the magnetic phase and the

piezoelectric phase, gives an opportunity for applications such

as actuators, sensors, and transducers.65–67

Recent advances in thin film deposition techniques such as

molecular beam epitaxy (MBE) and reflection high-energy

ED-assisted pulsed laser deposition (laser-MBE) make it im-

mensely possible to fabricate well-controlled layer-by-layer

heteroepitaxial perovskite films with a high degree of perfec-

tion. Such high-quality epitaxial perovskite films give an op-

portunity to study defect-free physical properties, to perform

intriguing interface studies, and even to create entirely new

crystalline structures in the form of artificial superlattices with

multifunctional properties.

Defects in the materials are one of the key factors that dimin-

ish the performance of the devices. For example, the presence of

dislocations and hence the surrounding strain field affect the

ferroelectric and the dielectric properties of a material.68–70

High density of misfit dislocation in PbZr0.52Ti0.48O3 grown on

SrRuO3-coated (001) SrTiO3 due to larger lattice mismatch dras-

tically decreases the switchable polarization and piezoelectric

coefficient d33.69 However, high-quality PbZr0.2Ti0.8O3 film

deposited on a carefully prepared SrRuO3-coated SrTiO3 (100)

substrate reduces misfit dislocation density and thereby increases

the polarization.71 A special substrate-preparation method is

adapted prior to deposition of the films to prevent the extension

of structural defects from the substrate to the films.72 High-

resolution transmission electron microscopy (TEM), a TEM

cross-sectional micrograph, and macroscopic polarization hys-

teresis measurement of defect-free PbZr0.2Ti0.8O3 are shown in

Figure 10 along with the results of the film prepared under

nonoptimized conditions.

The defect density and the remnant polarization values of

the film prepared under optimized growth conditions are su-

perior compared to the one prepared under nonoptimized

conditions.

Superlattice structures constructed using various perovskite

layers with different physical properties yielded artificial

structures whose properties are well controlled by interface-

and strain-mediated phenomena.74,75 The best example is the

BaTiO3/SrTiO3/CaTiO3 tri-component superlattice structure

grown with layer-by-layer control, compositionally abrupt in-

terfaces, and atomically smooth surface on an electrically con-

ducting SrRuO3 electrode by the high-pressure laser ablation

technique.74 The quality of such a film can be directly inferred

from the high-resolution TEM image and x-ray diffraction pat-

tern exhibiting superlattice peaks as shown in Figure 11.

Although SrTiO3 and CaTiO3 are not ferroelectric in the

bulk form, at RT, the resultant superlattice structures are ferro-

electric with 50% enhancement in polarization compared to a

pure BaTiO3 film grown under similar conditions. The en-

hancement in polarization is linked to the epitaxial strain

and interface effect. These results demonstrate the possible

enhancements of perovskite material properties that are highly

desirable in applications such as sensors and piezoelectric and

ferroelectric devices.

The heterostructure of ferromagnetic and ferroelectric pe-

rovskite gives a new opportunity to design multifunctional

materials with an ME coupling property. The graded hetero-

structure between the colossal magnetoresistant perovskite

manganite LaxSr1�xMnO376 and the ferroelectric PbZr1�xTixO3

with varying Zr content on a lattice-matched (100)SrTiO3 sub-

strate by the pulsed laser deposition technique gives a better

understanding of strain-related effect in these materials.77 It is

of note that the properties of these materials are controlled by

both extrinsic and intrinsic strain.76 A high-resolution cross-

sectional TEM micrograph of the heterostructure, magnetiza-

tion hysteresis loop of a single-layer La0.7Sr0.3MnO3, and the

heterostructure are shown in Figure 12(a), 12(b), and 12(c),

respectively.

The varying Zr content in the PbZr1 � xTixO3 in turn gradually

increases the epitaxial strain experienced by the La0.7Sr0.3MnO3

layers in the heterostructure with overall increase in lattice

parameters.77 These could alter the coercive field of the magnetic

layers. The consequence of this effect is visibly seen as a structure

with several steps at different values of the magnetic field in

the hysteresis loop of the six-layered heterostructure shown in

Figure 12(c), which is absent in the case of a single La0.7Sr0.3MnO3 layer (see Figure 12(b)). Hence, the magnetization can be

switched independently in each of the La0.7Sr0.3MnO3 layers in

such a heterostructure, which is the resultant of the strain-

engineered property. Interestingly, the heterostructure and the

superlattices formed between perovskite ferromagnetic manga-

nites and perovskite ferroelectric materials are believed to show

ME coupling.78,79 Even though such structures can be grown on

good lattice-matched substrates that will enable coherent growth

with flat interfaces, care must be paid to prevent interdiffusion

at the interfaces.80

4.03.3 Aurivillius Phase

A family of materials that can be derived from the perovskite

structure by careful selection of elements is the Aurivillius

phases. These materials are known for their ferroelectric and

dielectric properties with fairly high transition temperature

above RT. They have the general formula An�1Bi2BnO3nþ3,

where A¼Sr, Pb, Ca, Ba, etc., and B¼Mo, Fe, Mn, Ti, Ta, etc.

The compounds under this family of materials are well known

(a)

PZT

SRO

PZT

SRO

STO(b)

(d) (e)

(c)

0.004

0.002

0.000

-0.002

-0.004

-4 -2 0

Voltage (V)

Pol

ariz

atio

n (m

Ccm

-2)

Cur

rent

(A)

Cur

rent

(A)

2 4-100

-75

-50

-25

0

25

50

75

100

Pol

ariz

atio

n (m

Ccm

-2)

-100

-75

-50

-25

0

25

50

75

1000.003

0.002

0.001

0.000

-0.001

-0.002

-0.003

-4 -2 0

Voltage (V)2 4

STO

SRO

PZT

100 nm 200 nm

8 nm

Figure 10 (a) High-resolution TEM micrograph and (b) cross-section TEM micrograph of defect-free PbZr0.2Ti0.8O3/SrRuO3/SrTiO3. (c) Cross-sectionTEMmicrograph of defective PbZr0.2Ti0.8O3. Switching current and polarization hysteresis curves of (d) defect-free and (e) defective PbZr0.2Ti0.8O3 films.Reproduced from Vrejoiu, I.; Le Rhun, G.; Pintilie, L.; Hesse, D.; Alexe, M.; Gosele, U. Adv. Mater. 2006, 18, 1657–1661, with permission from Wiley-VCH; Vrejoiu, I.; Le Rhun, G.; Zakharov, N. D.; Hesse, D.; Pintilie, L.; Alexe, M. Philos. Mag. 2006, 86, 4477–4486, with permission from Taylor & Francis.

10

CaTiO31 nm

SrTiO3

BaTiO3

100

−5−4

−3−2

−10

+1+2

+3+4

+5

-3

-2

-1

0

+1

+2

+3-4101

102

Inte

nsity

(arb

itrar

y un

its)

103

104

105

106

107

20

STO001

STO002

302q (�)

40 50 60

Figure 11 (a) Cross-sectional Z-contrast image of compositionally abrupt interfaces in (SrTiO3)2/(BaTiO3)2/(CaTiO3)2 and its atomic structure. (b) XRDpattern of the superlattice structure confirming the long-range periodicity and the high crystallinity. Reproduced from Lee, H. N.; Christen, H. M.;Chisholm, M. F.; Rouleau, C. M.; Lowndes, D. H. Nature 2005, 433, 395–399, with permission from Nature Publishing Group.


−0.10−0.50

−0.25

0.00

0.25

0.50

LSMO

PZT 20/80

PZT 10/90

LSMO

(a)

LSMO

30 nmSTO substrate

−0.05 0.00Magnetic field μ0H (T)(c)

(b)

Mag

netiz

atio

n μ 0

M (T

)

Mag

netiz

atio

n μ 0

M (T

)

0.05 0.10

5 K100 K200 K

300 K

−0.10−0.6

−0.3

0.0

0.3

0.6

−0.05 0.00Magnetic field μ0H (T)

0.05 0.10

5 K100 K200 K

300 K320 K

Figure 12 (a) Cross-section HRTEM micrograph of an La0.7Sr0.3MnO3/PbZr1�xTixO3 heterostructure grown on STO(100). (b) The magnetizationhysteresis loops of a single La0.7Sr0.3MnO3 film (5 nm thin). (c) Magnetization hysteresis loops of the La0.7Sr0.3MnO3/PbZr1 � xTixO3 heterostructure.Reproduced from Vrejoiu, I.; Ziese, M.; Setzer, A.; Birajdar, B. I.; Lotnyk, A.; Alexe, M.; Hesse, D. Appl. Phys. Lett. 2008, 92, 152506–152508, withpermission from American Institute of Physics.


to be good candidates for nonvolatile ferroelectric memory

application.81 Figure 13 indicates the structure of an n¼2

Aurivillius phase (Bi3TiNbO9).

The crystal chemistry and the structural reasons for the

ferroelectric behavior of various materials in these complex

bismuth oxides are fairly well understood now. The crystal

chemistry and the stability of these compounds are well un-

derstood in terms of the number n.82 Small displacive pertur-

bations from the high symmetric prototype parent tetragonal

structure (I4/mmm) symmetry are ascribed to the observed

ferroelectric nature. The presence of the anisotropic lone-pair

Bi3þcation is understood to be the main catalyst for these

displacive perturbations.

Different members of this family of Aurivillius phases consist

of various combinations of similar structural units, (An�1BnO3nþ1)

2� perovskite blocks/layers and [Bi2O2]2þslabs.83 The

perovskite layers give the flexibility to engineer new materials

because of the compositional flexibility; the boundary slabs in

between the perovskite blocks are entirely made of Bi2O2 blocks.

The presence of the highly polarizable Bi is one of the main

reasons for the observed ferroelectric properties. Initial under-

standing of the Aurivillius systems suggested that only lone pair

cations such as Pb and Tl can replace the Bi in the Bi2O2 layers,

but this viewpoint has changed over time and it is now clear that

ions such as Sr, Ba, and La can indeed occupy the Bi2O2 layers.

This gives a greater flexibility in modifying the electrical polariza-

tions in these phases by suitable substitutions. Some of the

compositions have found great interest in electronic and piezo-

electric applications because of its high mechanical Q

(Qm¼12000).84 Thin film analogs of many of these Aurivillius

phases are of great interest due to potential applications in non-

volatile memories (ferroelectric random access memory,

FeRAM).85

Aurivillius phases include many orthorhombic ferroelectric

materials at RT. These are of commercial interest because of

their fatigue-free nature, low coercive field, and, in some cases,

for their relaxor behavior.86

The relation between the tolerance factor and the ferroelec-

tric Tc has been systematically investigated by Suarez et al.87

using TEM. Superlattice reflections due to tilting of octahedra

around the c-axis were identified and the systematic variations

of their intensities with the tolerance factor were followed from

the micrographs. It was found that the onset of octahedral

tilting and the Tc’s are strongly related in these Aurivillius

phases. Figure 14 indicates the systematic variation of Tc’s

with tolerance factor.

A systematic study of the various Aurivillius phase materials

has been done and expressed in terms of simple layered com-

pounds and different perovskites and representing them in

the form of an infinite series.88 The different tables presented

by Isupove88 indicate the ways in which the different layered

Aurivillius phases can be grouped based on the number

of perovskite blacks involved. The B-site ordering in the

perovskite layers not only changes the ferroelectric Curie


temperature but also leads to antiferroelectric ordering in some

of the compounds.

Partial replacement of Bi with Sr (the Ti site was equally

replaced with Nb to ensure charge neutrality) or La in the n¼3

Aurivillius layered phase was carried out to understand the

possibility of nonlone pair ions occupying the Bi2O2 layer.

This study has confirmed that Sr2þ ions indeed occupy the

Bi2O2 layers.89 Simple geometrical tolerance factor arguments

were used to rationalize the observed cation disorder, as shown

in Figure 15 in Bi and A-sites of the Bi2O2 slabs and the

900

800

700

600

500

400

300

200

100

00.96 0.97

CaBTO15CaBTO27

BTO12

SrBTO27 SrBTO15

SrBTO18

PbBT

PbBN

BaBTO27SrBNbO9

PbBTO27

Toleran

T c

Figure 14 Variation of ferroelectric temperature versus tolerance factor. Rep16, 3139–3149, with permission from the Materials Research Society.

Bi

Bi2O2 layers

Nb/Ti

O

Figure 13 Crystal structure of n¼2 Aurivillius phase, Bi3TiNbO9.

perovskite slabs, respectively. The incorporation of these non-

lone pair ions is explained to be due to the natural conse-

quence of size mismatch between the Bi2O2 layers and the

perovskite slabs.

High photocatalytic activity was observed in two new com-

positions of the n¼3 Aurivillius phase. The compounds of

interest for photo-catalysis applications were Bi3SrTi2TaO12

and Bi2LaSrTi2TaO12. Both these samples crystallize in the I4/

mmm space group and are found to have absorption in the

ultraviolet (UV)–visible region as shown in Figure 16(a).

Photocatalytic activity was measured by the observation of

decomposition of Rhodamine B solution containing the above

Aurivillius phase, at RT; Figure 16(b).90

Current interest in multiferroics propelled many groups to

look for new materials, which show the coexistence of magnetic

and electrical spontaneous polarization in single-phase

materials. Sharma et al.91 have investigated the possibility of

inducing magnetism in an n¼3 Aurivillius phase. Ru4þ, Ir4þ,and Mn4þ were incorporated into the M-site of Bi2�xSr2þx

(Nb/Ta)2þxM1�xO12 (x¼0.5). All the compositions exhibited

electrical properties similar to the parent compound. Mn-

doped compounds showed short-range ferromagnetism and

the other two doped systems exhibited signatures of antiferro-

magnetic exchange interactions. Clearly, no long-range magnetic

order is observed in any of the three samples. However, this work

indicates the potential use of Aurivillius phases as hosts for

new multiferroic materials.

Among the n¼4 members of the Aurivillius family,

(A)4B4O15 has also been investigated over the years. The A-site

can be occupied by a variety of cations such as Naþ, Kþ, Ca2þ,Sr2þ, Ba2þ, Pb2þ, Bi3þ, or Ln3þ and the B-site can be occupied by

Fe3þ, Cr3þ, Ti4þ, Nb5þ, W6þ, etc. It is quite possible to engineer

0.98

O15

bO9

PbBTO18

BaBNbO9

BaBTO15

ce factor (t)0.99 1.00

roduced from Suarez, D. Y.; Reaney, I. M.; Lee, W. E. J. Mater. Res. 2001,


the electrical polarization characteristics by carefully changing

the A- and B-site cations. For example, BaBi4Ti4O15 is a relaxor

ferroelectric exhibiting broad transition around Tc, but replacing

Ba ions with Ca, Sr, or Pb will render the system ferroelectric.92

The possible origins of these differences in the polarization

behaviors have been systematically analyzed by studying the

two limits of these n¼4 systems, CaBi4Ti4O15 (ferroelectric tran-

sitions at 790 �C) and BaBi4Ti4O15, in their single-crystal form.

In CaBi4Ti4O15, a significant deformation of the perovskite block

is observed in tune with the tolerance factor parameter expected

1.2

1.0

0.8

0.6

0.4

Ab

s.

0.2

0.0200 300

Bi2LaSrTi2TaO12

Bi3SrTi2TaO12

Bi3SrTi2TaO12

Bi2LaSrTi2TaO12

hv/ev3.0

0

10

20

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0

(ahv

)2 /(ev

)2

400

Wavelength/nm500 600 700

Figure 16 (a) Absorption spectra of Bi3SrTi2TaO12 and Bi2LaSrTi2TaO12 andZheng, H. J. Solid State Chem. 2010, 183, 361–366, with permission from El

Ti1/Nb1 O1

Bi1/Sr1

Bi2/Sr2[M2O2]

[A2B3O10]

c

a

O3

Ti2/Nb2 O5

O4

O2

Figure 15 An n¼3 Aurivillius phase with Sr ions occupying the Bi site.Reproduced from Hervoches, C. H.; Lightfoot, P. J. Solid State Chem.2000, 153, 66–73, with permission from Elsevier.

for a perovskite with small A-site cation such as Ca. Mixed Bi/Ca

atomic positions were found to exist in the perovskite blocks.

However, BaBi4Ti4O15 has been well studied recently and the

partial substitution of Bi for Ba in the (Bi2O2) slabs is found to be

a probable reason for the relaxor behavior.93

High-temperature sensing becomes very important in aero-

space and in certain industries such as automotive engineering.

To sense the strains and the vibrations, one needs high-

temperature stable electromechanical transducer materials or

high-temperature piezoelectrics. Aurivillius phases find enor-

mous interest in these fields where one needs high-temperature

piezoelectrics.94–96 They also find applications in the field of

materials for nonvolatile FeRAM devices.97,98 Photolumines-

cence properties also make these oxides useful as device

materials.99 Aurivillius phases are also finding applications as a

potential playground to find new ME multiferroics.100

4.03.4 Ruddlesden–Popper Structures

One of the intriguing materials that has shown great variations

in transport and magnetic properties is born out of a modified

perovskite structure called the Ruddlesden–Popper (RP) struc-

tures. The crystal structures of RP phases (A/O)(ABO3)n consist

of n perovskite blocks separated by a rock-salt (AO) interme-

diary. Blocks with n¼1, 2, 3, . . . are called one-layer, bilayer,

trilayer, etc. Blocks with n>3 are very difficult to synthesize as

a single-phase compound and are found in the literature. Good

structural investigation into the influence of size and crystal

symmetry on the stabilization of RP phases is discussed by

Besnosikov and Aleksandrov.101

The A-cations are in the cubo-octahedral anionic coordina-

tion (coordination number¼12) and are included in the pe-

rovskite stack. The A0 ions are located on the perovskite stack

boundaries with an intermediate block layer and have a nine-

fold coordination. The B-cations occupy the anionic octahe-

dral. Hence, the RP phases are the closest members of the

family of perovskite-related structures. The n¼1, 2, 3, and

n¼1 layered RP phases are depicted in Figure 17.

Most studies carried out recently on the RP phases are onMn-

based systems because of the colossal magnetoresistance (CMR)

and related magneto-transport phenomenon observed in these

1.0

0.8

0.6

0.4

0.2

0.0

C/C

0

Bi2LaSrTi2TaO12

Bi3SrTi2TaO12

No catalyst

Irradiation time/min0 20 40 60 80 100 120 140 160 180 200 220

(b) photocatalytic activity. Reproduced fromWang, D.; Tang, K.; Liang, Z.;sevier.


systems. Both the n¼2 and 1 members are well-known systems

that show colossal magnetoresistance behavior.102 Charge and

orbital ordering behavior has been observed in the single-layer

materials with different rare-earth ions constituting the A-site.

The n¼1 RP phase has A-cations occupying the crystallo-

graphic position between the perovskite-like stack and the

block layer. Most symmetric tetragonal n¼1 RP phases contain

two formula units per unit cell. The number of less symmetric

n¼1 RP phases is very less. The crystal structure and the mag-

netic structures of RP phases are complex and are subject to

controversy. It has been an impossible task to accurately de-

scribe the crystal structure due to the existence of pancake-like

defects. One such system, which has been reinvestigated to

understand the crystal structure, is Ca2MnO4.103 A coherent

coexistence of two crystallographic phases was found.

Nd1�xCa1þxMnO4 is an n¼1 member of the RP phase where

theNd-rich regions adopt a distorted orthorhombic structure and

the Ca-rich regions adopt the typical tetragonal structure.104

Charge-orbital ordered states were observed in the Nd-rich

y

xz x

y

(a) (b)

Figure 18 Crystal structure of Ba4Mn3O10 viewed (a) along [100] and (b) athe MnO6 octahedra. Reproduced from Zubkov, V. G.; Tyutyunnik, A. P.; BergeChem. 2002, 167, 453–458, with permission from Elsevier.

n = ∞ n = 1 n = 2

Sr(1)

Sr(2)

Figure 17 Crystal structures of n¼1-, 1-, and 2-layer Ruddlesden–Popper structures. Reproduced from Battle, P. D.; Rosseinsky, M. J. Curr.Opin. Solid State Mat. Sci. 1999, 4, 163–170, with permission from Elsevier.

regions with a fairly high transition temperature above RT. Super-

lattice reflections with a modulation vector q¼(1�x)a* are

observed in the electron diffraction patterns at low temperature.

The n¼2 member of the RP phase is the most studied

layered system, especially in the form of manganites. Detailed

magnetic andmagnetotransport studies have been carried out in

the last decade. HRTEM analysis on at least one composition,

(La,Ca)3Mn2O7, has been performed. It was found that the local

crystal structure indicates the presence of n¼3 and 4 phases and

these are prone to phase separation.105 Sr2IrO4 and Sr3Ir2O7 are

the n¼1 and 2 layer RP phases, respectively, with insulating

ground states.106,107 Moon et al.108 have investigated the elec-

tronic structures of the 1-, 2-, and infinite layer RP phases using

optical spectroscopy and first principles calculations and found

that the electron correlations play an important role in these 5d

systems. This result negates the general inference that the elec-

tron correlations are insignificant in 5d systems.108

Ca4Mn3O10 is an n¼3 member of the RP phase and de-

tailed magnetic and transport properties have been studied.109

This system goes to a G-type antiferromagnetic structure below

113.5 K and behaves like a quasi 2D antiferromagnet in the

temperature region above the Neel temperature. Parasitic

ferromagnetism is observed using neutron diffraction data.

Dzyaloshinskii–Moriya interactions from the orthorhombic

distortion of the crystal structure are ascribed to the cause of

the weak ferromagnetism observed in the sample. A finite

nonzero magnetic moment is observed well above the mag-

netic transition and is the cause of negative magnetoresistance

observed in these materials.

Attempts to completely replace the Ca ions by Ba (Ba4Mn3O10)

and retaining the n¼3 RP phase were unsuccessful.110 The

resulting structure consists of groups of three MnO6 octahedra

sharing faces to formMn3O12 groups and the trimers are linked

together by sharing the vertices (Figure 18). Long-range

z

long [010]. Filled circles represent Ba atoms and shaded octahedra arer, I. F.; Voronin, V. I.; Bazuev, G. V.; Moore, C. A.; Battle, P. D. J. Solid State


antiferromagnetism was observed in this compound at very

low temperatures (5 K).

Similarly, Sr4Mn3O10 does not crystallize in an RP phase.

However, the partial substitution of Mn by Fe or of Sr by Ca

leads to a stable n¼3 RP phase with tetragonal symmetry. A

minimum amount of x¼0.4 is needed in Sr4Mn3�xFexO10 to

stabilize the RP phase and a minimum of x¼2.85 is needed in

Sr4�xCaxMn3O10 by substituting the A-site. However, the de-

tailed microstructure studies using selected area ED patterns

indicate the intergrowth of n¼2 and 3 structures. Both the

substitute phases exhibit a spin glass-like magnetic phenome-

non and are semiconducting in nature.110

The effects of electron doping in n¼3 RP structures have

also been studied. The electron-doped 3D perovskites have

provided interesting insight into the physics of these materials

in the 3D perovskite manganites. Hence, it is only natural to

understand the effect of electron doping in the layered systems.

One such study was done on the system Ca4�xLaxMn3O10

(0<x<0.2).112 With small amounts of doping, the system

retains the G-type antiferromagnetic structure. Theoretical

studies investigated the effects of electron doping and have

found that the magnetic structure changes to either the A1-

type antiferromagnetic state or the A2-type ferrimagnetically

ordered state and the weak ferromagnetism observed in these

n¼3 systems is ascribed to the presence of the A2-type ferro-

magnetic state. However, only low levels of doping with La are

possible in these n¼3 systems and there are no large-scale

changes observed in the physical properties.113

Thorium-doped Ca4Mn3O10 also shows some universal

behavior in these doped three-layer systems with a prominent

ferromagnetic response.114 A significant MR of 66% at a

high field of 5 T and low temperature is also observed

(Figure 19).

Substituting other transition metal ions in the Mn site of

these n¼3 layer RP phases has resulted in some interesting

magnetic phenomenon. Introduction of pentavalent Mo into

the Mn site led to ferromagnetic and CMR behavior in the

system.115 Introduction of Nb in the Mn site has shown a strong

resemblance to the V-doped system.116 Tantalum doping of the

0

-10

-20

-30

-40

-50

-60

-70

0

-10

-20

-30

-40

-50

Temperature (K)

Field (T)

60 K

Hmin

20 K

MR

(%)

MR

(%)

0

-4 -2 0 2 4

50

x = 0.05

x = 0.10

x = 0.15

100 150 200 250 300

Figure 19 Magnetoresistant behavior of Th-doped Ca4Mn3O10.Reproduced from Lobanov, M. V.; Li, S. W.; Greenblatt, M. Chem. Mater.2003, 15, 1302–1308, with permission from American Chemical Society.

Mn site increased both the octahedral distortion and the unit

cell volume. Short-range ferromagnetic interactions were ob-

served, which were attributed to the double exchange interac-

tion between the Mn ions. At higher doping concentration,

evidence of cluster glass behavior was observed resulting from

the competing long-range AFM regions and FM clusters.

Nd4M3O10 (M¼Ni, Co) corresponds to the n¼3 member

of the RP phases.117 This is one of few non-manganate com-

pounds that have been investigated. Both Ni and Co compo-

sitions are found to be monoclinically distorted and the

lowering of symmetry is facilitated by the tilting of MO6 octa-

hedra. Nd4Co3O10 exhibits long-range antiferromagnetic

ordering below 15 K and the high-temperature paramagnetic

region exhibits two regions of different Curie constants. These

changes are attributed to the temperature-induced spin state

transitions similar to that observed in LaCoO3. A schematic

representation of the Nd4M3O10 is shown in Figure 20.

Incorporation of a diamagnetic element into the B-site

so that the super-exchange pathways can be reduced in a

cation-ordered arrangement was carried out. The two systems

tried were La4LiMnO8 and La3SrLiMNO8, both belonging to

the n¼1 RP phase. No long-range order was found from the RT

x-ray and neutron data. However, local 1:1 B-site ordering

was observed using magic angle spinning nuclear magnetic

resonance (MAS NMR) and electron microscopy. Electron mi-

croscopy studies indicate 1:1 Li:Mn order well developed in the

XY sheet of corner-sharing octahedra, but the 1:1-ordered

XY sheets were stacked randomly along z-axis (as seen in

Figure 21). Both the compounds were ordered antiferromagnet-

ically at low temperatures around 20 K. The hkl-dependent line

broadening seen in the low-temperature neutron diffraction

patterns indicate these systems to be 2D antiferromagnets.118

Some of the members of the RP family of compounds

do find applications as SOFCs. The studies on the RP

nickelate phases exhibit superior oxide ion conduction com-

pared to the conventional perovskite-based SOFCs.119 The

high-temperature stability of these layered oxide materials

as SOFC materials is still being investigated to understand

their commercial viability.120

4.03.5 Brownmillerites

One of the widely studied oxygen-deficient families of oxides is

the brownmillerite structure. This family of oxygen-deficient

perovskites is an interesting playground for a wide variety of

properties such as magnetoresistance, different magnetic order

parameters, ionic conductivity, and thermoelectric proper-

ties.121–124 The degree of oxygen deficiency will influence the

structure of the resulting compound. The manganite systems

with brownmillerite structure are particularly interesting be-

cause of the capability of tuning changes in the anion vacancy

and the oxidation states of the B-site cation.

A2BB/O5þx is the general formula for the one-layer brown-

millerite structure, with B ions occupying the octahedral site

and the oxygen deficiency leading to B/ being tetrahedrally

coordinated. A typical crystal structure of the brownmillerite

structure is given in Figure 22. The long-range vacancy order-

ing leads to a supercell with lattice parameters a¼21/2ap,

b¼4ap, and c¼21/2ap (where ap is the cubic perovskite lattice

constant). The structure of the single-layer brownmillerite can

Nd4

Nd2

Nd1

Nd3O7O7

O8O8

O6O6

O6

O4

O1O1

O1O1

O1O1

O1

O4

O4O4

O4

O6

O6

O8O8

O4

O10

O10

O10

O10 O10O10

O10

O10

O9O9

O9M3

M3

M2

M2

M4

M4

M4

M4

M1

M1O5

O5

O3

O3O3

O3

O3O2

O2O2 O2

O2

O2O2

O5

O5

Figure 20 Crystal structure of Nd4M3O10. Reproduced from Olafsen, A.; Fjellvag, H.; Hauback, B. C. J. Solid State Chem. 2000, 151, 46–55, withpermission from Elsevier.


be described as composed of alternating layers of AO, BO2, and

B/O along the unit cell b-axis. The layers are in the sequence

AO–BO2, AO, B/O, AO, BO2, AO, B/O,. . .. The anion vacancies

are ordered in the B/O layer.

Oxygen atoms in the B/O layers are shifted from their posi-

tions in the ideal cubic perovskite structure due to the coopera-

tive rotations of the tetrahedral, with the b-axis as the axis of

rotation (as given in Figure 23).125 Due to their cooperative

distortions and anion vacancies, the coordination number of

A ions will change to 10 (compared to CN¼12 in the ideal

perovskite structure). Two out of the ten A–O distances are

much longer than the other two A–O distances.

The B/O4 tetrahedra can have both clockwise and counter-

clockwise directions of rotations that alternate along the chains

of tetrahedra. These tetrahedral chains can adopt either the left-

or the right-handed orientations. The presence of L or R orien-

tations does change the A, B, or B/ coordination numbers. Both

the orientations are energetically similar and the probabilities

of occurrences are therefore the same.

Different space-group symmetries are observed depending

on the small difference in ordering of the tetrahedral chains

within a unit cell. Both right-handed (R) and left-handed (L)

orientations are possible. In the case of complete random

arrangement of left-handed and right-handed chains, a struc-

ture with Imma space group symmetry is obtained. A structure

with an I2mb space group is observed if there is only one type

of orientation present. A systematic alternation of L and R

chains leads to the Pnma space group symmetry. More compli-

cated patterns of L and R orientations have also been observed

resulting in space groups C2/c or Pcmb.126 A detailed report

on the correlation between the intra- and interlayer

chain correlations and their influence on the resulting

space groups is provided by Ramezanipour et al.127 Alternation

of L and R orientations in one layer of B/O4 has also been

observed leading to increase in the repeat period along the

c-axis.128,129

Ca2Fe2O5 is a well-known brownmillerite compound that

crystallizes in the Pnma space group and the oxidation state of

Fe ions is found to be 3þ from Mossbauer data.130 The ortho-

rhombic lattice of Ca2Fe2O5 undergoes a structural transition to

a body-centered structure with the space group I2mb around

950–1000 K. The ionic conductivity property of Ca2Fe2O5 in-

dicates that the level of ionic transport properties are lower than

that corresponding to cubic ferrite-based perovskites, as indi-

cated in Figure 24. The low concentration of oxygen vacancies

in perovskite-like layers is thought to be the reason for low ionic

conductivity exhibited in this system. The Fe3þ–O–Fe3þ interac-

tions in this compound make the system exhibit a long-range

G-type antiferromagnetic ordering.

Sr2Fe2O5 crystallizes in the Imma space group. Similar to

Ca2Fe2O5, this composition also exhibits long-range oxygen

vacancy ordering and a corresponding long-range antiferro-

magnetic ordering.131,132 However, the vacancy ordering has

distinct features indicating an ordering of L and R chains

BO2

R

R

L

L L

L

I2mb Pnma

BO2

BO2

B�O

B�O

Figure 23 Two types of tetrahedral chains and the brownmilleritestructures with I2mb and Pnma space group symmetry. Reproducedfrom Abakumov, A. M.; Kalyuzhnaya, A. S.; Rozova, M. G.; Antipov, E. V.;Hadermann, J.; Tendeloo, G. V. Solid State Sci. 2005, 7, 801–811,Elsevier.

O2

Fe1

O3

ba

c

Ca

O1

Fe2

Figure 22 Brownmillerite crystal structure (Ca2Fe2O5). Reproduced fromShaula, A. L.; Pivak, Y. V.; Waerenborgh, J. C.; Gaczynski, P.; Yaremchenko,A. A.; Kharton, V. V. Solid State Ion. 2006, 177, 2923–2930, withpermission from Elsevier.

q

z

p

x/y

20 Å

Figure 21 HRTEM image of La4LiMnO8, [100/010] projection andidealized [100] and [010] projection on the right side. Reproduced fromBurley, J. C.; Battle, P. D.; Gallon, D. J.; Sloan, J.; Grey, C. P.; Rosseinsky,M. J. J. Am. Chem. Soc. 2002, 124, 620–628, with permission fromAmerican Chemical Society.

0

-1

-2

-3

-4 p(O2) = 0.21 atm

104/T (K-1)

log

σ o (S

/cm

)

Ca2Fe2O5

Ca2FeAIO5

7.5 8.0 8.5 9.0 9.5 10.0

SrFe0.7Al0.3O3–δ

SrFe0.5Al0.5O3–δ

La0.3Sr0.7Fe0.8Ga0.2O3–δ

Figure 24 Temperature dependence of oxygen ionic conductivity ofCa2Fe2O5 compared with other brownmillerites. Reproduced fromShaula, A. L.; Pivak, Y. V.; Waerenborgh, J. C.; Gaczynski, P.;Yaremchenko, A. A.; Kharton, V. V. Solid State Ion. 2006, 177,2923–2930, with permission from Elsevier.



within the tetrahedral layers and having different ordering

stacking variations of these layers. Detailed electron micros-

copy studies indicate an ordering sequence of –L–R–L–R

within the tetrahedral layers. This arrangement provides the

shortest separation between chains of different orientations

and hence is the most favorable arrangement for the

mutual compensations of the opposite dipoles involved.

However, due to the absence of any energy saving preferred

in the L or R orientations, a different sequence of L and R

ordering is also seen in other tetrahedral layers. This leads

to the conclusion that the space group involved is Imma,

a disordered arrangement of L and R chains in different

tetrahedral layers.

Ca2Co2O5 and Sr2Co2O5 are other two brownmillerites

that have been investigated extensively. Of these, the structural

details of Ca2Co2O5 are not well understood. Ca2Co2O5 is of

interest as a good thermoelectric material.133 Sr2Co2O5, how-

ever, is reported to have a brownmillerite structure with the

space group Ima2 at RT and it undergoes several phase transi-

tions between RT and 1200 �C.134

Recently, there have been a few studies on compositions

containing two different B-site elements and investigations

into their structure and physical properties. Sr2FeCoO5 is one

such compound known to form a brownmillerite structure and

crystallizes in the Icmm space group. The single x-ray crystal of

Ca2FeCoO5 was reported recently and is one of the rare brown-

millerite systems that crystallize with the space group Pbcm.

There are two sets of octahedral and tetrahedral sites in

this crystal structure. The tetrahedral sites show almost perfect

Fe/Co ordering with octahedral sites indicating relatively

less Fe/Co ordering. This is the first compound showing inter-

layer cation ordering in the one-layer brownmillerite system.

Figure 25 AFM G-type structure of Ca2FeCoO5. Reproduced fromRamezanipour, F.; Greedan, J. E.; Grosvenor, A.; Britten, J.; Cranswick, L.M. D.; Garlea, V. O. Chem. Mater. 2010, 22, 6008–6020, with permissionfrom American Chemical Society.

The change in crystal structure does not lead to any major

difference in the overall magnetic structure/interactions and

it maintains a typical G-type AFM structure, as shown in

Figure 25,135 and observed in many brownmillerite com-

pounds. A reorientation of magnetic moments is observed at

different temperature ranges, as observed by neutron diffrac-

tion studies.

Sr2FeMnO5 and Ca2FeMnO5 are two other mixed B-site com-

positions that have been investigated recently.136 Sr2MnFeO5

synthesized in air showed no tendency of oxygen vacancy order-

ing and crystallized in a cubic structure with the space group

Pm–3m. On the other hand, when the sample was synthesized in

argon atmosphere, oxygen vacancy ordering was observed, but in

a small length scale of 5 A. The long-range structure remains the

same as in the sample synthesized in air. Ca2FeMnO5 forms a

long-range vacancy ordered brownmillerite structure. The pres-

ence of the larger Sr ion has an adverse effect on the oxygen

vacancy ordering so well established in Ca2FeMnO5. It is intrigu-

ing that Sr2Fe2O5 and Ca2Fe2O5 are both oxygen vacancy or-

dered brownmillerites and the destruction of vacancy ordering

with a bigger Sr ion in the A-site seems to occur only when both

Fe and Mn occupy the B-site. Ca2FeMnO5 indicates a typical G-

type antiferromagnetic structure, but the Sr2FeMnO5 systems

indicate local short-range G-type spin correlations. Ca2FeMnO5,

however, is a well-studied system with Mn predominantly in the

octahedral site and Fe almost fully occupying the octahedral site.

Mossbauer studies point at the antiferromagnetic ordering be-

tween the layered octahedral sublattice of Mn3þ and the chained

tetrahedral sublattice of Fe3þ ions.137

Ga as one of the B-site ions in the one-layer brownmillerite

structure renders the tetrahedral layers nonmagnetic, as tetrahe-

dral sites are preferentially occupied by Ga ions. Sr2MnGaO5þd

and Ca2MnGaO5þd are two such antiferromagnetic brownmil-

lerite oxides. Sr2MnGaO5 is found to crystallize in the Imcm space

group with L and R chains alternating with equal probability.138

A change in the oxygen content with the final composition as

Sr2MnGaO5.5 leads the system to crystallize with P4/mmm space

group symmetry and does not represent the true brownmillerite

structure. Both Sr2MnGaO5þd and Ca2MnGaO5þd show G-type

antiferromagnetic structure in their brownmillerite form and an

increasing amount of oxygen in the tetrahedral layer (dþ0.5);

the interlayer interactions turn ferromagnetic leading to a C-type

structure. Partial replacement of Ga by Al in Ca2MnGaO5 leads

the structure to a body-centered one with I2mb space symmetry,

with tetrahedral chains all being of the same type, where the

parent undoped system has intermixing of different chain

orientations.139

Considering the oxygen deficiency ordering in these brown-

millerites, it is a natural material to be investigated for its SOFC

characteristics. Among the materials investigated, Sr2Co2O5

and Sr2Fe2O5 are the most promising materials with high

oxygen ion conduction at RT with a very large charge transfer

of one electron per formula unit.140 Time resolved x-ray ab-

sorption fine structure measurements conducted recently indi-

cated that in Co systems the full charge transfer occurs, whereas

in the Fe-based system it does not. More importantly, the study

threw up the best possible space group to define the local

crystal structure in these two systems.141 Recent studies on

the thin films of Ca2Fe2O5 brownmillerite have provided

some insight into the oxygen conduction pathways, indica-

ting that the oxygen diffusion are highly anisotropic with


significant diffusion along the lateral direction of both tetra-

hedral and octahedral layers.142

A2B2B/O8þx forms the second group of the double-layer

brownmillerite structure.143 Here again, B cations are octahe-

drally coordinated and B/ ions occupy the tetrahedrally coor-

dinated sites. As one can imagine, here one tetrahedral layer

separates two octahedral layers as shown in Figure 26. Oxygen

vacancy ordering typically leads to lattice parameters with

a¼21/2ap, b¼3ap, and c¼21/2ap. Here again, the tetrahedral

chains can have both right- and left-handed orientations. We

also discuss the single layer brownmillerite structure-based

materials and observe their properties followed by a discussion

of the bi-layer materials.

Ca2.5Sr0.5Mn2GaO8 is one such double-layer system with a

nonmagnetic Ga tetrahedra separating out the magnetic Mn-

double octahedral layers.143 This shows a G-type antiferromag-

netic ordering below 120 K (Figure 27) and shows a 50%

magnetoresistance at a high-applied magnetic field. There has

been little incorporation of other ions in the A-site of this

double layer system and La-substituted systems have shown

ferromagnetic characteristics at low temperature.144

Brownmillerite materials find useful applications due to

their electronic and oxygen-ion conduction, proton conduct-

ivity,145,146 catalytic properties,146–149 photocatalysis,150 etc.

Because of the structural complexity and the ever-present oxy-

gen ion vacancy ordering, brownmillerite structures find natu-

ral uses in SOFCs and other catalytic applications.

y

zx

Figure 26 Crystal structure of Ca2.5Sr0.5Mn2GaO8. Reproduced from Battle,Condens. Matter. 2002, 14, 13569–13577, with permission from Institute of

4.03.6 Pyrochlores

Pyrochlores are oxides with the general formula A2B2O7 and

share a similar structural relationship with the mineral pyro-

chlore. The crystal chemistry of this structure150–155 is flexible

enough to allow different suitable elements in the A- and B-sites

that one can induce a wide range of interesting physical proper-

ties. Combination of A and B cations is found to be in general

A3þ and B4þ, but it is also possible to have a A2þ and B5þ system

even though the number of systems reported are really few. The

ability of the system to accommodate different elements in the

A- and B-sites makes these pyrochlores useful in a wide range of

applications. For example, they can be used as relaxor ferroelec-

trics, fast ion conductors, materials for display devices, optical

telecommunication components, biolabels, light emitters, ra-

dionuclide species host materials, etc.156–160 Finally, there is a

huge amount of interest in the versatile magnetic property

exhibited by many of these pyrochlores fundamentally brought

about by the peculiar crystal structure. A large part of research in

pyrochlore magnetism has been concentrated on the spin glass

and spin ice phases.

The term ‘pyrochlore’ comes from the mineral NaCaN-

b2O6F pyrochlore, reported in 1930 by Gaertner.161 Pyrochlore

means ‘green light’ because of the green color exhibited when

ignited by the mineral. This is cation-ordered anion-deficient

fluorite lattice. The majority of the synthesized pyrochlores

crystallizes with a cubic structure with Fd3m space group

GaO1

O3 O4

O5

O5 O2

O4Mn

GaOO

O3 OO4

O5

O5 O2

OMn

P. D.; Blundell, S. J.; Santhosh, P. N.; Rosseinsky, M. J.; Steer, C. J. Phys.Physics Publishing Ltd.

z

xy

Figure 27 G-type AFM magnetic structure. This schematic gives a view along [010]. Reproduced from Battle, P. D.; Blundell, S. J.; Santhosh, P. N.;Rosseinsky, M. J.; Steer, C. J. Phys. Condens. Matter. 2002, 14, 13569–13577, with permission from Institute of Physics Publishing Ltd.


symmetry. In a completely ordered A2B2O7, the superstructure

phase stability is determined by the A and B cation size ratio.

Typically, the ratio rA to rB lies between 1.46 and 1.79 in the

reported compounds with a stable pyrochlore structure. The

choice of A- and B-site ions has a big impact on the energetics

of vacancy formation and also on the order–disorder trans-

formations observed in these systems.161–163

A2B2O6O/ is the currently accepted formulation of an oxide

pyrochlore and A and B ions occupy the 16c and 16d sites in

the Fd3m space group. The oxygen ions O and O/ occupy the

48f and 8b sites, respectively (as indicated in Table 1). Both the

A- and B-sites form 3D arrays of corner-sharing octahedral as

shown in Figure 28. Alternately, the A- and B-site (16c and 16d

Table 1 Atom positions in a pyrochlore structure

Atom Wyckoff position Minimal coordinates

A 16d 0.5, 0.5, 0.5B 16c 0, 0, 0O 48f x, 1/8, 1/8O/ 8b 3/8, 3/8, 3/8

A-site B-site

Figure 28 Crystal structure of an ideal pyrochlore A2B2O7. Reproducedfrom Gardner, J. S.; Gingras, M. J. P.; Greedan, J. E. Rev. Mod. Phys.2010, 82, 53–107, with permission from American Physical Society.

sites) form layers stacked along the h111i direction. The coor-

dination numbers of the A- and B-site ions are decided by the

values of x of the O atoms. Values of x are generally between

0.320 and 0.345 where the A- and B-site ions are found to be in

distorted polyhedra. The distortion around the A-site ions is

considerably larger compared to the distortion around the B-

site ions. The A-site has a prominent axial symmetry and the

corresponding unique axis is along the h111i direction. This

structural feature has a lot to do with the physical properties

exhibited by the pyrochlore materials. Pyrochlore structures are

interpreted in two different ways. One way of interpretation is

to consider it as an ordered defect fluorite structure (CaF2). The

second way is to describe it as an interpenetration of a network

of corner-sharing B2O6 octahedra with zigzag chains of A2O/

running through the channel formed by the B2O6 octahedra

network.

The phase stability of pyrochlores with respect to the A- and

B-site ionic radii was presented by Subramanian and Sleight.165

Around half a dozen tetravalent ions are found to form stable

pyrochlore phases with different trivalent rare-earth ions in the

A-site. The Sn4þ ion is the only tetravalent ion, which forms a

stable pyrochlore structure with all the rare-earth ions. Smaller

B-site tetravalent ions such as Mn need an extra factor to stabilize

the pyrochlore phase, the parameter being high pressure.

Recently, Lu Cai et al.166 have tried to extend the idea of using

the tolerance factor concept, so widely used in perovskites, to the

pyrochlore phases. The authors introduced two tolerance factors

t1 and t2 and calculated the values of these two terms for over 300

known pyrochlores. The tolerance factors t1 and t2 are defined as

t1 ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffix� 1

4

� �2qþ 1

32ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffix� 1

2

� �2qþ 1

32

rA þ rOrB þ rO

where is x is the x-parameter of the 48f site and


t2 ¼ a3

ffiffiffi3

p

8 rA þ rOð Þ

The inclusion of the x parameter in the tolerance factor

makes the factor more precise compared to the tolerance factor

concept proposed earlier by Isupov.167 Values of t1 were found

to vary between 0.83 and 1.07 with the majority centering

around 0.90 and 0.99. However, the t2 values are always found

to be less than 1 compared to the maximum value of 1.07 found

for t1.

Some interesting trends were observed when t2 values were

plotted against the ratio of A- and B- cation size, rA/rB. t2 is

found to decrease with increasing rA/rB, whereas t1 does not

show any trend. Figure 29 shows the plots of t2 versus rA/rB.

The normalized dielectric difference is found to have clear

variation with t2, as shown in Figure 30, indicating its impor-

tance in predicting dielectric properties with respect to the

structural parameter.

0

2

Δen

4

6

0.88

Bi1.65Zn0.35Ti1.65Nb0.35O7

Bi1.5Zn0.5Nb0.5Ti

Bi1.5Zn0.6

Bi1.

Bi1.5Zn0.667Sn

Bi1.5Zn0.833Sn0.5Nb1.1

Bi1.5Zn0.9167Sn0.25Nb1.333O

Bi1.5Zn0.92Nb1.5O6.92

Bi1.5ZnNb0.5Ta

0.89 0.90 0.91

Figure 30 Variation of normalized dielectric difference of various pyrochlore pNino, J. C. J. Mater. Chem. 2011, 21, 3611–3618, with permission from The R

1.00.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04 Ambient atmosphereAmbient atmosphere (displacive disorder)High pressure

1.2 1.4 1.6rA/rB

t 2

1.8 2.0 2.2 2.4

Figure 29 Variation of tolerance factor t2 with the ratio rA/rB.Reproduced from Cai, L.; Arias, A. L.; Nino, J. C. J. Mater. Chem. 2011,21, 3611–3618, with permission from The Royal Society of Chemistry.

Magnetic frustration is an intriguing field of study for re-

searchers working with magnetic materials because of the rich

physics involved in it. Understanding the various types of in-

teractions involved is an exciting challenge that is worth taking

up. There are different classes of frustration, the geometric frus-

tration and the random frustration. Geometric frustration can

best be explained by the example of Ising spins (that can point

only in up or down directions) interacting antiferromagnetically

and occupying the vertices of an equilateral triangle. Whenmany

such triangles form an edge-sharing triangular lattice, the frustra-

tion leads to a disruption of a long-range antiferromagnetically

ordered state. However, random frustrations are formed by the

systems developing nontrivial spatial correlations to resolve the

frustrations as found in the formation of stripe-like features in

cuprate superconductors. In the case of pyrochlores, the previ-

ously mentioned structural details indicate that it is made up of

arrays of A or B polyhedra. If the A or the B ion is magnetic and

its nearest neighbor interaction is antiferromagnetic, then the

structural implications make the system a perfect playground

for frustrated magnetic interactions leading to novel exotic quan-

tum mechanical effects at very low temperatures. The observed

magnetic phenomena in these pyrochlores include spin glasses,

spin liquids, disordered and ordered spin ice, superconductivity,

Kondo-like behavior, etc.

Spin glass behavior is observed in Mo-based pyrochlore

oxides. For example, Y2Mo2O7168 and Tb2Mo2O7

169 are the

two prominent compounds studied by many groups, with

the former getting the bulk share of attention. Frequency-

dependent measurements of the Ac susceptibility show the

classical shift in spin glass freezing temperature with frequency,

as shown in Figure 31.170 Thermo-remanent magnetization

measurements on these systems further confirm the classical

spin glass nature in Y2Mo2O7. The 89Y NMR experiments

indicate a distribution of Y environments brought about by

the local lattice.171 Muon spin relaxation data analysis172 in-

dicates that both spin and lattice degrees of freedom contribute

to the spin freezing mechanism. The origin of spin glass nature

in Y2Mo2O7 is still not completely understood.

Tb2Mo2O7 is a bit more complex compared to Y2Mo2O7

because of the contribution of Tb moments to the magnetic

1.5O7

67Nb0.833TiO7

5Zn0.833Nb1.167Ti0.5O7

Bi1.5Zn0.9167Nb1.333Ti0.25O7

Nb0.8333O7

67O7

7

Bi1.5Zn0.5Nb0.5Zr1.5O7

Bi1.5Zn0.5Nb0.5Ce1.5O7Bi1.5Zn0.5Nb0.5Sn1.5O7O7

Bi1.5ZnNbTa0.5O7

BiZnTiNbO7

0.92t2

0.93 0.94 0.95

hases with the new tolerance factor t2. Reproduced from Cai, L.; Arias, A. L.;oyal Society of Chemistry.

Figure 32 (a) Ice structure and the corresponding spin ice structure in(b) pyrochlores. Reproduced from Gardner, J. S.; Gingras, M. J. P.;Greedan, J. E. Rev. Mod. Phys. 2010, 82, 53–107, with permission fromAmerican Physical Society.

180.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

20 22 24

Temperature T (K)

χ� (a

rb. u

nits

)

26

1085 Hz

Y2Mo2O7h = 10 Oe

311 z108 Hz37 Hz3.7 Hz

28

Figure 31 AC susceptibility data of Y2Mo2O7 exhibiting spin glassbehavior. Reproduced from Miyoshi, K.; Nishimura, Y.; Honda, K.;Fujiwara, K.; Takeuchi, J. J. Phys. Soc. Jpn. 2000, 69, 3517–3520, withpermission from Physical Society of Japan.


interactions that is, here two geometrically frustrated sub-lattices

are being dealt with. Tb2Mo2O7 also shows unusual positive and

negative MR at low- and high-applied fields respectively, the

reasons of which remains unclear. Tb2Mo2O7 shows spin glass

behaviors with a freezing temperature at 25 K. However,

Eu2Mo2O7173 is metallic and ferromagnetic below 50 K. Doping

the Eu site with La shows a re-entrant spin glass behavior at 22 K.

Nd, Sm, and Gd in the A-site makes these molybdenum pyro-

chlores ferromagnetic,174 whereas Tb and Dymakes it spin glass.

Along with this, the presence of nonmagnetic ions such as Y and

La in the A-site consistently gives rise to spin glass behavior with

no clear dependence on the average ionic radius, contrary to the

effects in many perovskite manganates.

In an ice structure, the oxygen atoms form a perfectly ordered

crystalline structure with the hydrogen atoms being paired with

two oxygen atoms. With each pair of oxygen atoms, it will form

a short covalent bond and a long hydrogen bond. Four protons

form the nearest neighbor ions with each oxygen atom and

forms a tetrahedron with protons at the corners. Pairwise repul-

sive interaction in this configuration cannot be satisfied at the

same time and leads to a frustrated system Minimization of the

total energy is achieved by having two protons nearer to

the oxygen atom and the other two farther away. This is termed

as ‘two in’ and ‘two out’ configuration. In a pyrochlore lattice,

the rare-earth ions can achieve this ‘two in and two out’ config-

uration magnetically with similar geometrical constraints and

hence are termed ‘spin ice’ material as indicated in Figure 32.

Ho2Ti2O7 and Dy2Ti2O7 are the two pyrochlore phases that

exhibit ‘spin ice’ behavior.

A2Ti2O7 (A¼Ho3þ or Dy3þ)175,176 was the first system to

be found to exhibit spin ice behavior. Ho3þ and Dy3þ with

a large magnetic moment of about 10mB contribute to the ‘two

in–two out’ configuration. These systems have magnetostatic

dipole–dipole interactions that are ferromagnetic in nature

and form the main source leading to frustrated interaction.

The absence of dipolar interactions between the Ho or Dy

ions would have resulted in a long-range antiferromagnetically

ordered structure at low temperatures. Ramirez et al.175 carried

out detailed magnetic specific heat measurements and gave

concrete evidence for the existence of spin ice behavior in

Dy2Ti2O7. Ho2Ti2O7 is the second compound that was found

to show spin ice behavior. Anomalously large hyperfine in-

teractions between the electronic and nuclear spins for Ho give

rise to a nuclear specific heat Schottky anomaly at 0.3 K, lead-

ing to rapid rise in specific heat at very low temperatures. Once

the nuclear contribution was taken into account, the electronic

specific heat data indicated a magnetic specific heat deficit

pointing toward spin ice behavior.177

Ho2Sn2O7 and Dy2Sn2O7 are naturally the other systems

that exhibit similar characteristics as the spin ice systems.178 In

Ho2Sn2O7, the local environment around the Ho ions is sim-

ilar to that found in the spin ice system. A newmacroscopically

degenerate phase called ‘kagome spin ice’ is achieved by apply-

ing a magnetic field along [111] direction of Dy2Ti2O7 and

Ho2Ti2O7. In the ‘two-in and two out’ spin ice phase, one spin

is pinned by the applied magnetic field leading to a lowering of

the number of possible ground states. When the field is more

than 0.3 kOe, 40% residual entropy is seen compared to the

zero field spin ice. This phase is the Kagome spin ice phase and

magnetization measurements have indeed proved the exis-

tence of such a phase.179 Catelnovo et al.180 have described

in a groundbreaking analysis the possibility of observing mag-

netic monopoles in a frustrated pyrochlore system such as

Dy2Ti2O7. They have theoretically shown how the excitations

(or defects that locally violate the ice rule) above the ground

state ‘two-in two-out’ manifold are indeed magnetic mono-

poles. The spin ice state can be well described by networks of

aligned dipoles that resemble solenoidal tubes or ‘Dirac

Strings’ (as shown in Figure 33). When these strings are bro-

ken, the resulting defect at the boundary looks like magnetic

monopoles. This has been experimentally observed by diffused

neutron scattering experiments with the aid of a symmetry

breaking magnetic field. Dy2Ti2O7 is the first artificially

grown structure in which the experimental evidence of mag-

netic monopoles was discovered.181

There are large number of papers available on the pyro-

chlore magnetic properties, ranging from long-range ordered

magnetic structures to spin glasses, spin ice, and spin liquid

structures. A recent excellent review on the magnetic properties

of pyrochlore structures by Gardner et al. has covered the entire

system of magnetic pyrochlores in detail.182

Dielectric properties of a number of pyrochlore-based com-

pounds have been reported over the years. Some of the pyro-

chlores with Bi as the A-site ion have been investigated in great

detail. Bi3/2ZnNb3/2O7 with a dielectric constant of 150 and aeof �400 ppm �C�1 is by far the most extensively studied

0

250

300

350

400

450

100

1 KHz BSTN10 KHz100 KHz1000 KHz

200 300

T/�C

w t gd

400 5000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Figure 34 Temperature dependence of dielectric constant anddielectric loss in Bi3/2ZnNb3/2O7. Reproduced from Du, H.; Shi, X. Appl.Phys. Lett. 2010, 97, 52901–52903, with permission from AmericanInstitute of Physics.

0.00

10

20

30

40

(b)

(a)0

0

10

20

1 2

B

A

3

Cd2Re2O7

Cd2Re2O7

0.5T (K)

T (K)

Ce/

T (m

JK-2

mol

Re)

Res

istiv

ity (μ

Ωcm

)

1.0

μ0H = 0 T

Figure 35 (a) Resistivity and (b) specific heat measurements onCd2Re2O7 confirming the superconducting transition. Reproduced fromHanawa, M.; Muraoka, Y.; Tayama, T.; Sakakibara, T.; Yamaura, J.; Hiroi, Z.Phys. Rev. Lett. 2001, 87, 187001–187004, with permission fromAmerican Physical Society.

Figure 33 Pictorial representation of monopoles by the breaking ofDirac strings. Reproduced from Morris, D. J. P.; Tennant, D. A.; Grigera,S. A.; Klemke, B.; Castelnovo, C.; et al. Science 2009, 326, 411–414, withpermission from the American Association for Advancement of Science.


among these. Here, the structure is such that one expects half of

the Zn and all Bi ions to occupy the A-site. These compounds,

which arise from the Bi2O3–ZnO–Nb2O3 phase diagram, ex-

hibit a fairly large dielectric constant, low dielectric losses, as

well as tunable (compositionally) temperature coefficient of

capacitance. Compositionally modifying the above system is

found to induce a relaxor ferroelectric character. For example,

partially substituting Zn with Sr and Nb with Tin Bi3/2ZnNb3/

2O7 gives rise to relaxor ferroelectricity, as indicated in

Figure 34.183 The shift in the permittivity peak to higher tem-

peratures with increasing frequency indicates relaxor behavior.

Cd2Re2O7 is a pyrochlore belonging to the class of materials

called ‘ferroelectric metals’. This pyrochlore undergoes a cubic

to tetragonal structural phase transition at 200 K, which

satisfies the Anderson and Blount184 account of the Landau

theory to explain the possibility of a metallic ferromagnet.

The temperature dependence of elastic moduli was measured

across the cubic to tetragonal phase transition and helped to

rule out strain as an order parameter in this compound. The

evidence of elastic moduli points to the possibility of a loss of

inversion center and hence, making Cd2Re2O7 a candidate for

the ‘metallic ferroelectric’.185

Cd2Re2O7 is the first pyrochlore compound to be reported

to show superconductivity,186 having a bulk superconducting

transition at 1 K. The resistivity and specific heat measurements

showing superconducting transitions are given in Figure 35.

High-temperature data indicate a distinct structural phase tran-

sition, which has its implication of the resistivity and magnetic

properties with no long-range magnetic order.

Ho2Ti2O7 is another compound that has been reported to

have a ferroelectric character,187 but the understanding of this

effect is poor. Pyrochlore-structured oxides are known to be


good oxide ionic conductors with few of them having conduc-

tivity values comparable with yttria-stabilized zirconia (YSZ).

Pyrochlores are materials with intrinsic vacancies and having

no possibility for dopant–vacancy association. The most stable

intrinsic defect in pyrochlore is found to be a Frenkel pair,

involving a vacant 48f position and an interstitial at an 8b

site188 and oxygen ion diffusion occurs through jumps be-

tween 48f sites in the structure. Ionic conductivity in these

disordered pyrochlores depends on the energy of formation

of the Frenkel pair and mobility of the ions. The ionic conduc-

tivity in these disordered pyrochlores is found to depend on

the synthesis conditions of these systems.

Sm2Zr2O7 is a perfectly ordered pyrochlore and Dy2Zr2O7 is

disordered. The systematic substitution of the Sm site by Dy was

investigated by Sayedatal to understand the changes in ionic

conductivity by disorder induced by Dy substitution. The activa-

tion energy of ionic conduction was found to increase with

increasing Dy content or increasing disorder. The increased dis-

order tends to decrease the oxygen ion mobility and hence

higher activation energy and low ionic conduction.189

Significant proton conductivity properties were observed in

few pyrochlores. For example, Sm2Sn2O7 and Sm2Sn2�xYxO7

were found to shown significant proton conduction properties

under wet conditions. Similarly, La2Zr2�xYxO7 was also inves-

tigated and was found to be a good candidate as proton

conductors.190,191

x

z

y

Figure 36 Spinel structure indicating intertwining octahedra andtetrahedra.

A-site Co2+

(a) (b)

t2

Si

i

SjP

j

t2g

e

eg

CoO4

P = a eij X[Si XSj ]

B-site Cr3+

CrO6

Figure 37 (a) Structure of CoCr2O4 and spin states of the cations and(b) spin canting between two sites and the resultant polarization.Reproduced from Yamasaki, Y.; Miyasaka, S.; Kaneko, Y.; He, J.-P.;Arima, T.; Tokura, Y. Phys. Rev. Lett. 2006, 96, 207204–207207, withpermission from American Physical Society.

4.03.7 Spinels

Oxide spinel structures have been of great importance due to their

soft ferromagnetic properties and their use in high-frequency

applications. They are also fundamentally important because of

the rich spin/charge/orbital ordering phenomenon.192 One of

the most studied spinels is Fe3O4193 and more fundamentally

for the Verwey transition observed in these systems. The Verwey

transition in these systems has been reinvestigated along with

the observation of charge ordering observed in several perovskite

manganates.

The crystal structure of a spinel is well known. A spinel is

represented by AB2X4, with A and B generally being transition

metal ions and X oxygen. A typical structure is given in

Figure 36. B-cations form a pyrochlore lattice leading to geo-

metric frustration. The A2þ ions form a diamond lattice and the

cations occupying the A-site can either be magnetic or be non-

magnetic. Almost all compounds investigated in the oxide

spinel family have magnetically active transition metal ions

in the B-site. Studies over the decades have concentrated on

modifying themagnetic properties by careful selection of A and

B-site ions.

Recent interest in these oxide spinels has been instigated by

the explorations of new materials with multiferroic character,

more so from the studies of mutiferroic properties of distorted

perovskites such as TbMnO3. The collinear to spiral spin tran-

sition observed at Tc also sees the emergence of a ferroelectric

state. This ferroelectric feature observed in conjunction with

the spiral spin transition is in tune with the spin–current

model for the magnetic ferroelectricity proposed by Katsura

et al.194 The efforts to see systems with spiral components of

magnetizations, which can give rise to ferroelectricity and

which also have spontaneous/homogeneous magnetization,

led to chromate spinels with conical spin structures. There

have been quite a few reports recently on the magnetic spinel

oxides exhibiting dielectric anomaly or ferroelectric properties

closely related to their magnetic interactions.195–198

A ferroelectric state is observed in the ferromagnetic spinel

CoCr2O4 as the system shows a transition to a conical spin

order below 25 K. The crystal structure and electronic configu-

ration of CoCr2O4 are shown in Figure 37. The spin current

model of Katsuya et al. describes the relation between polari-

zation P and the neighboring spins (Si and Sj) and their relative

orientation as given by the relation in Figure 37(b). The vector

eij denotes the connecting vector between the two spin sites and

is in the direction of the spontaneous current flow and a is a

proportionality constant determined by the spin exchange and

spin–orbit interactions. The family of chromate spinels

MCr2O4 (M¼Mn, Fe, or Co) can show conical spin structures

that can show the polarization arising out of the spiral com-

ponents of magnetizations as well as the spontaneous mag-

netic state. CoCr2O4, for example, shows a ferrimagnetic


transition at Tc¼93 and 26 K goes through a transition to a

conical spin state.199

Studies on large single crystals of CoCr2O4 by Yamasaki

et al.200 confirmed the spontaneous polarization with a

[110} direction as the spontaneous polarization direction nor-

mal to the magnetization easy axis [001] and to the propaga-

tion axis110 of the transverse spiral component. Figure 38(a)

and 38(b) shows the magnetic-field dependence of the spon-

taneous polarization at 27 and 18 K, above and below the

ferroelectric transition, along with the corresponding magneti-

zation curves. This study indicated clearly that P always re-

versed upon the reversal of M clearly exhibited by the

sequential scan of the magnetic field between þ0.2 and

–0.2 T at 18 K (below the ferroelectric transition). This study

confirmed that the relative directional relation between P and

M is kept intact even after the reversal ofM once the ME-cooled

multiferroic state is established.

However, FeCr2O4 exhibited a larger polarization

(Figure 39) compared to CoCr2O4 and this is attributed to

the structural distortion induced by the presence of the Fe2þ

Jahn–Teller ion.201 Optical conductivities studies have indi-

cated the lowering of symmetry in the chromate spinels with

a Jahn–Teller ion occupying the A-site.202

Vanadate spinels have found attention due to the geometric

frustrated interactions and the possibility of orbital ordering.

At low temperature, a structural transition from cubic to tetrag-

onal symmetry203 is observed. The magnetic structures are

complicated and hence create a lot of interest. The magnetic

structure is antiferromagnetic along xy chains, but has ##"" alongxz and yz chains. This ##"" spin ordering (chains) is quite similar

to the magnetic structure observed in the multiferroics, E-type

-0.4

18 K (+Ec, -Hc)

18 K

18 K (+Ec, +Hc)

(1)

(3)

(2)

(b)

(a)

MHz

27 K (+Ec, +Hc)

27 K

-2

-1

0

1

-0.1

20

(2) -0.5 TT (K)

(3) 0.5 T

-2

-1

0

1

2

25

0

0.1

2

-0.2 0Magnetic field (T)

Pol

ariz

atio

n (m

Cm-2

)M

agne

tizat

ion

(mB

per

f.u.

)

P (

mCm-2

)

0.2 0.4

Figure 38 Magnetic field dependence of (a) magnetization and (b)electrical polarization at 27 and 18 K of the CoCr2O4 spinel. Reproducedfrom Yamasaki, Y.; Miyasaka, S.; Kaneko, Y.; He, J.-P.; Arima, T.; Tokura,Y. Phys. Rev. Lett. 2006, 96, 207204–207207, with permission fromAmerican Physical Society.

manganite HoMnO3204 and Ca3CoMnO6,

205 opening up the

possibility of observing multiferroicity in vanadate spinels too.

CdV2O4 is a geometrically frustrated spinel, which exhibits a

ferroelectric state at the transition to a collinear antiferromag-

netic ground state. In this system, the ferroelectricity is driven by

local exchange striction and not by the spiral magnetic structure

observed in chromate spinels. The ##"" spin ordering is found to

give rise to an electronic instability that leads to a V–V dimeriza-

tion and thereby the formation of short and long V–O bonds.

The oxygen bonded to ", " V (or #, # V) becomes inequivalent

and upon ionic relaxation gives rise to orbital ordering. The V–V

dimerization and the orbital ordering stabilize the spontaneous

polarization. The polarization observed in these vanadate spinels

due to the exchange striction mechanism is larger than that due

to spiral magnetism observed in chromate spinels.206 However,

no polarization has been observed in ZnV2O4 and MgV2O4, the

reasons for which are not understood.

4.03.8 Delafossites

The family of delafossite structures, AMO2, where A is an

aliovalent ion (Ag, Cu, etc.) and M is in the MIII state has

created interest as successful materials for p-type transparent

conductors.207 Each A-site ion is coordinated with two oxygen

atoms linearly forming O–A–O dumbbells parallel to the

c-axis. Each oxygen in these dumbbells is coordinated with

three MIII ions such that MIII-centered octahedra form MO2

layers lying parallel to the ab plane. Figure 40 indicates the

layered structuring of delafossite structures. They are further

classified into rhombohedral 3R or hexagonal 2H based on the

stacking of alternate layers.

The O–A–O dumbbells are interweaved with MO6 blocks

along the c-axis as shown in Figure 40. The dumbbells and

MO2 blocks can provide separate conduction paths for holes

and electrons, respectively. Theoretical calculations indicate that

these oxides have high hole mobility due to highly disperse

valence bands.208 Most importantly, the highest p-type transpar-

ent conducting oxide with highest conductivity reported

0-40

-20

0

P (μ

Cm-2

)

P (μ

Cm

−2)

20

40

20 40 60 80T (K)

T (K)

00

1

2

3

25 50

120 kV m−1 _14T

CoCr2O4

FeCr2O4

75 100

10

40.5

42.0

43.5

20 30 40

125 150

100 120

+200 kV / m-1

-200 kV / m-1

200 kV / m-1 _14T

140

Figure 39 Polarization curve of Fe2Cr2O4 after poling and undermagnetoelectric cooling. Inset figures show the polarization FeCr2O4 andCoCr2O4 under magnetoelectric cooling conditions. Reproduced fromSingh, K.; Maignan, A.; Simon, C.; Martin, C. Appl. Phys. Lett. 2011, 99,172903–172905, with permission from American Institute of Physics.

10-3

4 5 6 7 8

1000/T (K-1)

Con

duc

tivity

(Scm

-1)

9

CuY1-xCaxO2

CuSc1-xMgxO2

CuCr1-xMgxO2

10 11 12

10-2

10-1

100

101

102

Figure 41 Conductivity versus temperature plot of CuCr0.95Mg0.05O2.Reproduced from Nagarajan, R.; Draeseke, A. D.; Sleight, A. W.; Tate, J.J. Appl. Phys 2001, 89, 8022–8025, with permission from AmericanInstitute of Physics.

a

B

b

CAg

Z

X Y

c

A

A

Figure 40 The layered structure of a delafossite oxide. Reproducedfrom Dong, H.; Li, Z.; Xu, X.; Ding, Z.; Wu, L.; Wang, X.; Fu, X. Appl. Catal.B: Environ. 2009, 89, 551–556.


(220 S cm�1) to date is a delafossite oxide, CuCr0.95Mg0.05O2

(Figure 41).209 Doping of a trivalent metal site with a divalent

ion improves the conductivity in these materials.210 However,

the trends observed were not related to the structural variations

on the basis of ionic radii concepts.211,212

Cu-based delafossite CuAlO2 has been found to have high

activity and a very long lifetime in the gas phase oxidation of HCl

to Cl2 making it a good catalyst.213 CuMO2 (M¼Al, Cr, Fe, Fa, Y,

In) are reported as p-type semiconductors with a small indirect

band gap and are black in colour.212 Hence, the compounds

CuMO2 are inefficient as photocatalysts to photo-oxidize organic

compounds. AgAlO2 however has a relatively larger band gap

that can absorb UV light and can act as a photocatalyst. AgMO2

(M¼Al, Ga, and In) have higher band gaps to absorb visible

light and to generate photocatalytic activity as shown by their

influence on the degradation of Rhodamine B and methyl

orange.214 Magnetoelectric effects were observed in CuFeO2

where the application of very high magnetic fields induces the

polar phase215 and the substitution of the Fe site with Al (very

small doping levels) also creates the polar phase in the absence

of magnetic fields.216 The parent CuFeO2 is a triangular lattice

antiferromagnet (frustrated). Substituting with Rh in the Fe

site also gives rise to a spin-driven ferroelectricity.217 The

magnetoelectric effects found in these delafossite systems essen-

tially occur at very low temperatures (�10 K).218,219

4.03.9 Conclusion

In this chapter, we have tried to incorporate some of the recent

developments in the field of oxide materials. The area of oxide

materials is so vast that it is impossible to put together all the

oxide systems reported in the last few years because of the sheer

number of publications in the area. The field of simple transi-

tion metal oxides such as ZnO is so big that one needs a

separate chapter for the same. Here, we have tried to bring

out some of the interesting physical properties exhibited by

these few complex oxide materials. Oxide materials will con-

tinue to create huge interest in the materials research commu-

nity because of the richness in the physics and chemistry

involved in the properties exhibited by them. For a related

chapter in this Comprehensive, we refer to Chapter 2.09.

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