hexagonal close-packed structurelawm/263 2.pdfperovskite structure: abx 3 tolerance factor (t): 2( )...
TRANSCRIPT
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HEXAGONAL CLOSE-PACKED STRUCTURE
An HCP crystal is a close-packed structure with the stacking sequence ...ABABAB...
AB
To construct:
1st layer: 2D HCP array (layer A)
2nd layer: HCP layer with each sphere placed in alternate interstices in 1st layer (B)
3rd layer: HCP layer positioned directly above 1st layer (repeat of layer A)
A
…ABABABAB…
A
B
A
HCP is two interpenetrating simple hexagonal
lattices displaced by a1/3 + a2/3 + a3/2 70
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HCP STRUCTURE
• not a Bravais lattice
• each sphere touches 12 equidistant nearest neighbors (CN = 12)
• structure has maximum packing fraction possible for single-sized spheres (0.74)
Orientation alternates
with each layer
Six in plane, six out-of-plane
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HCP STRUCTURE
• ideal ratio c/a of 8/ 3 1.633
• unit cell is a simple hexagonal lattice with a
two-point basis
(0,0,0)
(2/3,1/3,1/2)
a
a
Plan view
• {0002} planes are close packed
• ranks in importance with FCC and BCC
Bravais lattices 72
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HCP STRUCTURE
• about 30 elements crystallize in the HCP form
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CUBIC CLOSE-PACKED STRUCTURE
A CCP crystal is a close-packed structure with the stacking sequence ...ABCABC...
To construct:
1st layer: 2D HCP array (layer A)
2nd layer: HCP layer with each sphere placed in alternate interstices in 1st layer (B)
3rd layer: HCP layer placed in the other set of interstitial depressions (squares, C)
AB C
4th layer: repeats the 1st layer (A)
…ABCABCABC…
It turns out that the CCP structure is just
the FCC Bravais lattice!
stacking
of HCP layers
along
body diagonals
74
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CCP STRUCTURE
• CN = 12, packing fraction 0.74
• {111} planes are close packed
• 4 atoms in unit cell
Plan view
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• most common are HCP and CCP
CLOSE-PACKED STRUCTURES
• an infinite # of alternative stacking sequences exist
Example: silicon carbide has over 250 polytypes
e.g., 6H-SiC
stacking sequence …ABCACB…
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STACKING FAULTSStacking faults are one or two layer interruptions in the stacking sequence that
destroy lattice periodicity
perfect FCC
ABCABCABC
faulted FCC
ABCBCABC
The stacking fault is an example of a planar defect
A
B
C
A
B
C
A
B
C
A
B
C
B
C
A
B
C
[111]-
[001]-
[110]-
[110]
e.g., an <110> projection of an FCC lattice:
missing
plane
of atoms
• stacking fault energy γ ~100 mJ m-2
• also results in a linear defect called a dislocation77
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EXAMPLE InAs nanowires - <110> projection
Caroff, P. et al. Nature Nanotechnology 4, 50 - 55 (2009).78
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• CCP and HCP have very similar lattice energies
• no clear cut trends
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Nature 353, 147 - 149 (12 Sep 1991)
Rare Gases: Ne, He, Ar, Kr, Xe (CCP)
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gold nanocrystals
X. M. Lin
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ANOTHER VIEW OF CLOSE PACKING
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. It was
reviewed by a panel of 12 referees; the panel reported in 2003, after 4 years of
work, that it was “99% certain” of the correctness of the proof, but couldn’t
verify all of the computer calculations. Hales and Ferguson (his student)
received the Fulkerson Prize for outstanding papers in the area of discrete
mathematics in 2009.
In 2003, Hales announced that he would pursue a formal proof of the
Conjecture that could be verified by computer. He estimated that the proof
would be finished by 2023…but it was announced complete on Aug 10, 2014!!!!
http://en.wikipedia.org/wiki/Kepler_conjecture
In January 2015 Hales and 21 collaborators published "A formal proof of the Kepler
conjecture". The proof was accepted in 2017.
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PACKING FRACTIONS
3
3
4( )
3 2 0.5236
a
fractiona
3
3
atoms
cell
4 24 ( )
3 4 2 0.74056
V
V
a
fractiona
2a
a
CCP (and HCP)
BCC
2
4
aradius
3
3
4 32 ( )
3 4 0.6802
a
fractiona
3
4
aradius
SC
74%
68%
52%
The fraction of the total crystal volume that is occupied by spheres
84
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85
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1atoms
cell A C
m nA
V N V
n: number of atoms/unit cell
A: atomic mass
VC: volume of the unit cell
NA: Avogadro’s number
(6.023×1023 atoms/mole)
Calculate the density of copper.
RCu = 0.128 nm, Crystal structure: FCC, ACu= 63.5 g/mole
n = 4 atoms/cell, 3 3 3(2 2 ) 16 2CV a R R
3
8 3 23
(4)(63.5)8.89 /
[16 2(1.28 10 ) (6.023 10 )]g cm
8.96 g/cm3 in the literature
DENSITY CALCULATION
86
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INTERSTICIAL SITES IN CP STRUCTURES
A large number of ionic structures can be regarded as built of CP layers of anions
with the cations placed in interstitial sites
for every anion, there is 1 Octahedral site and 2 Tetrahedral sites
87
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Octahedral holes
coordinates:
½00
0½0
00½
½½½
= O site
cavities have <100>
orientation
88
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Tetrahedral holes in CCP T+ sites:
¾¼¼
¼¾¼
¼¼¾
¾¾¾
T- sites:
¼¼¼
¾¾¼
¼¾¾
¾¼¾
cavities have <111> orientation89
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Holes in HCP
O sites:
2/3,1/3,1/4
2/3,1/3,3/4
T+ sites:
1/3,2/3,1/8
0,0,5/8
(0,0,0)
(1/3,2/3,1/2)
T- sites:
0,0,3/8
1/3,2/3,7/8
a2
a1
90
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LOCATION OF OCTAHEDRAL HOLES
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LOCATION OF TETRAHEDRAL HOLES
(3/8 of a unit cell directly above/below each anion)
92
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SIZE OF OCTAHEDRAL CAVITYOnly cations smaller than the diameter of the cavity can fit
without forcing the anion lattice to expand
M = cation
X = anion.
The cavity radius is 41%
of the anion radius
2 2M Xa r r
2 2 Xa r
from cell edge
from face diagonal
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SIZE OF TETRAHEDRAL CAVITY
The cavity radius is
22.5% of the anion radius
The tetrahedral holes are twice as
numerous but six times smaller in volume
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EUTACTIC STRUCTURES
Structures in which the arrangement of ions is the same as in a close packed array
but the ions are not necessarily touching
Within certain loose limits (given by the radius ratio rules), cations
too large to fit in the interstices can be accommodated by an
expansion of the anion array
• anions don’t like to touch anyway
• modern techniques show that, in many cases, anions (cations) are
not as large (small) as previously thought
• we still describe eutactic structures as CCP or HCP lattices
with ions in some fraction of the interstitial sites
95
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CRYSTALS THAT CAN BE DESCRIBED IN
TERMS OF INTERSTITIAL FILLING OF A
CLOSE-PACKED STRUCTURE
96
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SOME EUTACTIC CRYSTAL STRUCTURES
Variables:1) anion layer stacking sequence: CCP or HCP array?
2) occupancy of interstitial sites
97
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98
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NaCl (ROCK SALT, HALITE) STRUCTURE
→
→
→
99
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POLYHEDRAL REPRESENTATION
• shows the topology and indicates interstitial sites
• tetrahedra and octahedra are the most common shapes
•
•
•
100
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Very common (inc. 'ionics', 'covalents' & 'intermetallics' )
• Most alkali halides (CsCl, CsBr, CsI excepted)
• Most oxides / chalcogenides of alkaline earths
• Many nitrides, carbides, hydrides (e.g. ZrN, TiC, NaH)
ROCK SALT - OCCURANCE
101
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COMPLEX ION VARIANT OF ROCK SALT
•
•
102
2
2S
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→
→
→
ZINC BLENDE (ZnS, SPHALERITE)
103
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ZINC BLENDE
• bonding is less ionic than in rock salt
• common for Be, Zn, Cd, Hg chalcogenides (i.e., ZnS, ZnSe, ZnTe)
• common for III-V compounds (B, Al, Ga, In with N, P, As, Sb)
104
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DIAMOND STRUCTURE
Fd3m
→
105
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FLUORITE (CaF2) & ANTIFLUORITE (Na2O)
→
→
→106
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ALTERNATIVE REPRESENTATIONS
107
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FLUORITE / ANTIFLUORITE
• origin of the term “fluorescence”
(George Stokes, 1852)
• fluorite common for fluorides of large,
divalent cations and oxides of large
tetravalent cations (M2+F2 and M4+O2)
• antifluorite common for
oxides/chalcogenides of alkalis (M2O)
108
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FLUORESCENT MINERALS
http://en.wikipedia.org/wiki/Fluorescence= fluorite109
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COMPARING NaCl, ZnS, Na2O
110
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111
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Li3Bi EXAMPLE
112
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NiAs STRUCTURE
→
→
→
2
113
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114
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•Transition metals withchalcogens, As, Sb, Bi e.g. Ti(S,Se,Te); Cr(S,Se,Te,Sb); Ni(S,Se,Te,As,Sb,Sn)
115
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WURTZITE (ZnS) STRUCTURE
→
→
→116
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117
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118
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HCP VERSION OF CaF2?
No structures are known with all Tetra. sites (T+ and T-) filled in HCP- i.e. there is no HCP analogue of the Fluorite /Anti-Fluorite structure
Why?
The T+ and T- interstitial sites above and below a layer of close-packed spheres in HCP are too close to each other (distance = 0.25c) to tolerate the coulombic repulsion generated by filling with like-charged ions.
Face-linking is unfavorable120
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RUTILE STRUCTURE (TiO2)
→
→
→
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ANATASE STRUCTURE (TiO2)
→
→
→
122
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RUTILE AND ANATASE
→
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CdI2 STRUCTURE
→
→
→124
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125
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• Iodides of moderately polarizing cations; bromides andchlorides of strongly polarizing cations;
e.g. PbI2, FeBr2, VCl2
• Hydroxides of many divalent cations e.g. (Mg,Ni)(OH)2
• Di-chalcogenides of many quadrivalent cations e.g. TiS2, ZrSe2, CoTe2
CdI2 - OCCURANCE
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CdCl2 STRUCTURE
Chlorides of moderately polarizing cationse.g. MgCl2, MnCl2
Di-sulfides of quadrivalent cations e.g. TaS2, NbS2 (CdI2 form as well) 127
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FormulaType and fraction
of sites occupied CCP HCP
AB All octahedralNaCl
Rock Salt
NiAs
Nickel Arsenide
Half
tetrahedral
(T+ or T-)
ZnS
Zinc Blende
ZnS
Wurtzite
A2B All tetrahedralNa2O Anti-Fluorite
CaF2 Fluoritenot known
A3BAll octahedral
& tetrahedralLi3Bi not known
AB2
Half octahedral
(Alternate layers
full/empty)
CdCl2 (Cadmium Chloride) CdI2 (Cadmium Iodide)
Half octahedral
(Ordered
framework
arrangement)
TiO2 (Anatase)CaCl2
TiO2 (Rutile)
AB3
Third octahedral
Alternate layers 2/3 full/empty
YCl3 BiI3128
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PEROVSKITE STRUCTURE ABX3 (CaTiO3)
→
→
→129
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PEROVSKITE CONNECTIVITY
3D network of corner-sharing octahedra
Network of face-sharing cuboctahedra
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Perovskites: the most widely studied oxide structure
• Wide range of chemistries possible
- thousands of examples known
• Cubic, tetragonal, and orthorhombic symmetries are common
- high Tc cuprate superconductors
- Colossal Magneto-Resistance (La,SrMnO3)
- fast ion conduction (Li+, O2- in SrSnO3), batteries, fuel cells
- mixed electronic/ionic conduction, fuel cells
- oxidation/reduction catalysts
- ferroelectric / piezoelectric ceramics (BaTiO3, Pb(ZrTi)O3)
- important mineral structure in lower mantle (MgSiO3, pyroxene)
- frequency filters for wireless communications : Ba(Zn1/3Ta2/3)O3
Unique properties of perovskites
131
(CH3NH3)PbI3organic-inorganic
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Perovskite Structure: ABX3
Tolerance factor (t):
2( )
A X
B X
r rt
r r
A
t Effect Likely structure
> 1 A cation too large to fit in interstices
Hexagonal perovskite
0.9 - 1.0 ideal Cubic perovskite
0.71 - 0.9 A cation too small Orthorhombic perovskite
< 0.71 A cation same size as B cation Possible close packed lattice
B
X
132
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PEROVSKITES
BaTiO3: Ba2+ r = 1.56 ÅTi4+ r = 0.68 ÅO2- r = 1.26 Å
KNbO3: K+ 1.65 ÅNb5+ 0.78 Å
LiNbO3: Li+ 1.06 ÅNb5+ 0.78 Å
t = 1.03 - tetragonal
t = 1.01 - orthorhombic
t = 0.81 – trigonal
Most perovskites contain distorted octahedra and are not cubic, at least at lower
temperatures. These distortions give perovskites a rich physics.
symmetry at 25°C
LiNbO3 : ferroelectricity, Pockels effect, piezoelectricity, photoelasticity,
nonlinear optical polarizability
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V m-1
C m-2
Reading: West Ferroelectricity
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DI- , PARA- , AND FERROELECTRICS
dielectric polarization paraelectric polarization ferroelectric polarization
• linear: P = ε0χeE
• no P without E
P = ε0χeEP : polarization (C/m2)ε0: vacuum permittivity – 8.85 x 10-12 C2 N-1 m-2
χe: electric susceptibility (unitless)E : electric field (V/m, or N/C)
dipole moment: p = qd = αE
p
polarization: P = Σp/V
response of atom to applied E field
• nonlinear
• no P without E
• residual (zero-field) polarization• reversible direction of residual P• very large susceptibilities
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WHY IS BaTiO3 FERROELECTRIC
ferroelectric phase transition
> 120°C cubic, not FE< 120°C tetragonal, FE
transition occurs at the Curie temperature, Tc
~0.1 Å displacement
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die
lect
ric
cons
tant εr = χe+ 1
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FERROELECTRIC HYSTERESIS LOOPS
saturation polarization, Ps
remnant polarization, PR
coercive field, EC
dipoles aligned “up”
dipoles aligned “down”
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ORDERED ELECTRIC DIPOLE PHASES
ferroelectric (BaTiO3)
antiferroelectric (PbZrO3)
ferrielectric (Bi4Ti3O12)
• net spontaneous polarization in only certain direction(s)
• antiparallel ordering below Tc
• parallel ordering below Tc
E field can induceferroelectric state
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CURIE TEMPERATURE
Thermal energy destroys the ordered electric dipole state. The temperature above
which this order-disorder phase transition occurs is the Curie temperature, Tc.
Above Tc, the material is often paraelectric.
orderedF / AF
randomizedorientation
PNote: These curves omit the “spikes” in Pat Tc
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PHASE DIAGRAMS
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Doped La2CuO4 was the first (1986) High-Tc Superconducting Oxide (Tc ~ 40 K)
Bednorz & Müller were awarded a Nobel Prize
La2CuO4 may be viewed as if constructed from an ABAB... arrangement of
Perovskite cells - known as an AB Perovskite!
K2NiF4 STRUCTURE (La2CuO4)
Many “complex” structures are composed of simple, familiar building blocks.
The high-Tc copper oxide superconductors are an example.
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ALTERNATE VIEWS OF La2CuO4
We may view the structure as based on:
1. Sheets of elongated CuO6 octahedra, sharing only vertices
2. Layered networks of CuO46-, connected by La3+ ions
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COMMON STRUCTURAL FORM
• Common structural motif of vertex-linked CuO4 squares• This motif occurs in all the high-TC superconducting copper oxides • The structures differ in the structure of the 'filling' in the 'sandwich‘of copper oxide layers - known as Intergrowth Structures
Cations formFCC with O2-
interstitials
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Y1Ba2Cu3O7: THE 1,2,3 SUPERCONDUCTOR• the first material to superconduct at LN2 temperature, Tc > 77 K• YBa2Cu3O7 can be viewed as an Oxygen-Deficient Perovskite
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Two types of Cu sites: 1) Layers of CuO5 square pyramids2) Chains of vertex-linked CuO4 squares
POLYHEDRAL REPRESENTATION OF YBCO
CuO2
BaO
CuO
BaO
CuO2
Y
CuO2
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SPINEL STRUCTURE AB2O4 (MgAl2O4)
→
→
→
•
•
•
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149
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SPINELS - OCCURANCE
Aluminium spinels: Spinel – MgAl2O4, after which this class of minerals is namedGahnite - ZnAl2O4
Hercynite - FeAl2O4
Iron spinels: Magnetite - Fe3O4
Franklinite - (Fe,Mn,Zn)(Fe,Mn)2O4
Ulvöspinel - TiFe2O4
Jacobsite - MnFe2O4
Trevorite - NiFe2O4
Chromium spinels: Chromite - FeCr2O4
Magnesiochromite - MgCr2O4
Others with the spinel structure: Ulvöspinel - Fe2TiO4
Ringwoodite - Mg2SiO4, an abundant olivine polymorph within the Earth's mantle from about 520 to 660 km depth, and a rare mineral in meteorites
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No CFSE
CRYSTAL FIELD STABILIZATION ENERGY
In transition metal compounds, d electron effects such as crystal field stabilization energy (CFSE) can be important in determining structure.
e.g. MF2 compounds (high spin rutile)
crystal field splitting diagrams
Δtetra = (4/9)Δoct
Δoct =
Δtetra =
CFSEoct = (0.4 × #t2g – 0.6 × #eg) Δoct
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CATION SITE PREFERENCES IN SPINELS
Normal - [A]tet[B2]octO4 Inverse - [B]tet[AB]octO4
In the absence of CFSE effects: 2,3 spinels tend to be normal (MgAl2O4)4,2 spinels tend to be inverse (TiMg2O4)
In 2,3 spinels, CFSE favors the following: 1) Chromium spinels (Cr3+) are normal
2) Magnetite (Fe3O4) is inverse b/c Fe3+ has zero CFSE, while Fe2+ prefers oct.
3) Mn3O4 is normal b/c Mn2+ has no CFSE
γ = fraction of A in oct. sites
The larger CFSE of metal ions in octahedral sites is sometimes an important factor in determining spinel structures (normal vs inverse).
γ = 0 is normal, γ = 1 is inverse
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CORUNDUM STRUCTURE (α-Al2O3)
→
→
→
•
153