structural r espon s e to p ressure i nduced e lectronic t ransitions in tm-compounds

30 May, 200930 May, 2009 ERICE 2009ERICE 2009 11

StructuralStructural r responesponsse to e to ppressure ressure iinduced nduced

eelectroniclectronic ttransitionsransitions in in TM-compoundsTM-compounds

Moshe Paz-Pasternak, Tel Aviv University, Moshe Paz-Pasternak, Tel Aviv University, ISRAELISRAEL

Beware of false knowledge; it is Beware of false knowledge; it is more dangerous than ignorancemore dangerous than ignorance

George Bernard ShawGeorge Bernard Shaw

30 May, 200930 May, 2009 ERICE 2009ERICE 2009

What types of electronic transitions What types of electronic transitions may lead to structural phase may lead to structural phase transition in TMC’s?transition in TMC’s?

o highhigh to to lowlow spin transitions spin transitionso Intra-band overlapIntra-band overlap; ; the the Mott-HubbardMott-Hubbard

correlation breakdowncorrelation breakdown..o Cationic inter-bandCationic inter-band overlapoverlap; ; valence valence

exchangeexchangeo and and more….more….


– Appropriate electronic spectroscopy methods with Appropriate electronic spectroscopy methods with radiation that can be transmitted through radiation that can be transmitted through diamonds such asdiamonds such as::

– K-K-edge X-rays of the TM-ion to be used for XAS, XANES, edge X-rays of the TM-ion to be used for XAS, XANES, XES, EXAFS, etc.XES, EXAFS, etc.

– Mössbauer spectroscopy in iron-containing samplesMössbauer spectroscopy in iron-containing samples..– Optical spectroscopyOptical spectroscopy

- and- and– Wires for resistance and other electrical measurementsWires for resistance and other electrical measurements


TheThe d d-shell -shell ((Hund’s rules)Hund’s rules)

Fe3+Fe3+(LS)) 5 ↑↓ ↑↓ ↓ 1/2 3Fe2+Fe2+(LS)) 5 ↑↓ ↑↓ ↑↓ 0 3Fe3+(LS)) 5 ↑↓ ↑↓ ↓ 1/2 3


P

Fe3+

Fe2+

The high spin state is unstable at high-The high spin state is unstable at high-pressurepressure

P“Spin

crossover”


Fe3+(LS)) 5 ↑↓ ↑↓ ↓Fe3+(HS)) 5 ↑ ↑ ↑ ↑ ↑

Radius of TMRadius of TMHS HS > Radius of TM> Radius of TMLS

0 10 20 30 40 50 60 70 80 90

0.80

0.88

0.96

1.04

Pressure (GPa)

V/V

0

EuFeOEuFeO33

Fe3+(LS)Fe3+(HS)


Mott Hubbard insulator Mott Hubbard insulator

The strong on-site Coulomb repulsion produces an energy gap, within the 3d band, known as the Mott-Hubbard gap (U).

The insulating gap may also arise from a finite ligand-to-metal p-d charge-transfer energy Δ. In the case of Δ <U we have a Charge-Transfer insulator.

1 1MHn n n nd d d d

1CTn nd d L (L - ligand hole)

- electronic configuration of the TM ion nd

U >

> U

B


electronic/magnetic electronic/magnetic consequences consequences of Mott-Hubbardof Mott-Hubbard correlation- correlation-breakdownbreakdown

correlated statescorrelated states Uncorr. Uncorr. statesstates

insulatorinsulator metallicmetallic

OddOdd numbenumbe

r of r of spinsspins

HSHS LSLS

S S ≠ 0≠ 0 S S ≠ 0≠ 0 paramagneticparamagnetic

EvenEven numbenumbe

r of r of spinsspins

S S ≠ 0≠ 0 S S = 0= 0paramagneparamagne

ticticdiamagnediamagne

tictic

0S


Mössbauer Mössbauer spectroscopyspectroscopy

That’s why we can use absorbers with

diam. <0.1 mm

The nuclear scattering The nuclear scattering cross-section of cross-section of

5757Fe(14.4 keV) gamma-Fe(14.4 keV) gamma-rays is ~ 10rays is ~ 109 9 barns!barns!

currently the best experimental method at the atomic currently the best experimental method at the atomic scale for studying magnetism at very high pressures scale for studying magnetism at very high pressures

Rudolf.Nobel 1961


±±vvdetectordetector

Nuclear Nuclear resonant resonant scattererscatterer

Synchrotron Synchrotron monochromatimonochromati

c beamc beam


Mössbauer spectroscopy Mössbauer spectroscopy for pedestriansfor pedestrians

The hyperfine interaction in The hyperfine interaction in 5757FeFe Effect of pressure upon HEffect of pressure upon HHypHyp

The Isomer ShiftThe Isomer Shift Determining relative abundance Determining relative abundance

of componentsof components


tt1/2 1/2 ~ 5x10 ~ 5x10-7-7 sec!!! sec!!!

ΓΓ ~ 0.5 ~ 0.5 μμeV!!!eV!!!

Two quadrupole-split components

Magnetic splitting

±3/2

±1/2

1/2

QS

+3/2+1/2-1/2-3/2

-1/2

+1/2

~µHhyf

ΓΓ, , tt1/21/2

57Co

e.c decay

14. 4 keV

22ΓΓ

I=1/2I=1/2

II**=3/2=3/2

The Hyperfine Interaction in The Hyperfine Interaction in 5757FeFe

57Fe


The effect of Pressure upon the The effect of Pressure upon the Hyperfine FieldHyperfine Field

With <With <LLzz > = > = 0 0 the the orbital term is quenched orbital term is quenched and and HHOO = = 0.0.

With pressure increase With pressure increase HHOO →→ 00

57

3

( Fe) 22 ( )

2 ( )

hf S O

S z

zO B

d

H H H

H S T

LH Tr -

= ±

= -

= m

(2 )z zBS Lmµ m ±

*HO is P-dependent!

“S” spin term, “O” orbital term.

The Fe magnetic-moment:


ΔΔRR//RR is a nuclear constant. is a nuclear constant.ρρss(0) is the (0) is the ss-electrons density at the -electrons density at the nucleusnucleus

57( ) )Fe (0s

RIS

R

Decrease in Decrease in ISIS Increase in the Increase in the densitydensity at the at the vicinity vicinity of the of the FeFe site site

Isomer shift; an unique atomic-scale densitometer


Determining the component-abundance Determining the component-abundance nnii

1 12 2

.i i i jn const f A A n

n An A

=

=

å


Structural Response to PI Structural Response to PI electronic transitions in Feelectronic transitions in Fe2+ 2+

compoundscompounds

FeO (FeO (wwüüstitestite) ) NaCl structureNaCl structure

FeXFeX22 (X=Cl, I) (X=Cl, I)

Fe(OH)Fe(OH)22

CdICdI2 2

structurestructure


-15 -10 -5 0 5 10 15

0.90

0.95

1.00

Velocity mm/s

60 GPa

90 GPa

P (GPa)

Re

lati

ve

tra

ns

mis

sio

n

100 GPa

80 1200.0

0.5

1.0

120 GPa

abundance

T=300 K

HS > LS starting at ~ 90 GPaHS > LS starting at ~ 90 GPa

No symmetry or appreciable volume No symmetry or appreciable volume change ever detected.change ever detected.LS

HS

NaCl structureNaCl structure

Experimental proof of Hund’s ruleExperimental proof of Hund’s rulePPmechanicalmechanical > P > PCoulombicCoulombic


-10 -5 0 5 100.92

0.96

1.00

velocity (mm/s)

inte

nsi

ty

Mg0.9

Fe0.1

O

5 K

78 GPa LSdiamagnetic

0 GPa HS antiferromagnetic

MgMg0.90.9FeFe0.10.1OO

0 10 20 30 40 50 60 70 80

0.4

0.6

0.8

1.0

0.0

0.5

1.0

Mg0.9

Fe0.1

O

low

-spi

n re

gim

e

high-spin regime

coex

iste

nce

regi

me

IS (

mm

/s)

Pressure (GPa)

QS

(m

m/s

)

low-spin data

high-spin data


0 10 20 30 40 50 60 70

1.30

1.35

1.40

1.45

1.50

1.55

1.60

1.65

1.70

LP phaseIP phaseHP phaseDecompression

c/a

Pressure (GPa)

0 10 20 30 40

0

50

100

IS (

mm

/s)

Hhf(T

)T

N(K

)A

bund

ance

Pressure (GPa)

LP phase IP phase HP phase (metallic)

0

20

40

Hhf = H

S

Hhf = H

S-H

OHP phase

LP phase

IP phase

0.4

0.6

0.8

1.0

(d)

(c)

(b)

(a)

0

100

200

300

0 10 20 30 40 50 60 7045

50

55

60

65

70

75

80

85

90

95

100

Insulating (MH) phase

LP phaseHP phaseDecompression

Vol

ume

(Å3)

Pressure (GPa)

metallic phase-8 -6 -4 -2 0 2 4 6 8

7 GPa

18 GPa

Inte

nsity

Velocity (mm/s)

20 GPa

23 GPa

10 K 40 GPa

26 GPa

30 GPa

FeIFeI22


FeFe((OOHH))22

Parise et alParise et al


T >> TN

Paramagnetic Fe2+

T << TN

anti-ferromagnetic

Fe2+

2 3Fe Fe


0 10 20 30 400.0

0.2

0.4

0.6

0.8

P

P

Fe+

3 ab

un

d.

P(GPa)

H Lateral displacement (Parise et al)Fe3+ abundance


32

22( ) ( )PFe OH eFe OH

5 10 15 20

#

G L +

+

***

11

2

20

12

0011

111

0

10

210

1

10

0

00

1

28

18

11

5

1.5

P(GPa)

Inte

nsi

ty

2 (deg)

No change in structure!


Conclusion

the orientation-disorder of the O-H dipoles the orientation-disorder of the O-H dipoles caused by the pressure-induced OH----HO caused by the pressure-induced OH----HO coulomb repulsion, and, coulomb repulsion, and,

to the exceptional small electron binding to the exceptional small electron binding energy of Feenergy of Fe2+2+

32

22( ) ( )PFe OH eFe OH

The irreversible oxidation process is attributed to:

Within the HP band-structure of Fe(OH)2 a new, localized band is formed populated by the “ousted” electrons


Structural response to PI Structural response to PI electronic transitions in Feelectronic transitions in Fe3+ 3+

oxidesoxides

FeFe22OO33 ( (hematitehematite))

R R FeOFeO33 ((RR= rare-earth iron = rare-earth iron

perovskites)perovskites)

CuFeOCuFeO3 3 (delafossite)(delafossite)


FeFe22OO33 a correlation breakdowna correlation breakdown

5 10 15 20

(a) 2.9 GPa

Inte

nsity

Diffraction Angle 2

*

(b) 46.0 GPa

Fig. 3

*

(c) 70.0 GPa

(d) Rh2O

3 II-type calc.

123

114

= 0.4246 Å

132

024

204

131

220

004

113

122

103/

211

200

11202

0

111

002

Rutile > RhRutile > Rh22OO33 II II

ΔΔVV/V/V00 = = 0.10.1


FeFe22OO33 a catastrophic correlation a catastrophic correlation

breakdownbreakdown

5/ 2S

0S

INSULATOR-METAL TRANSITION COLLAPSE OF MAGNETISM


• Correlation breakdown triggersa 1st-order structural phase transition

• Similar transitions are observed in GaFeO3 and FeOOH, pointing to a structural instability of (Fe3+O6) species atP > 50 GPa.

Summary


All R FeO3 (R3+ rare earth ) undergo HS>LS transition at ~ 40 GPa

At P > 100 GPa they remain paramagnetic (<S>≠0) down to 4K.


IM takes place at ~ 120 GPa


A 1st (or 0th) order structural phase transition occurs at the HS>LS crossover with 3-5% volume reduction but with no symmetry change!

No hysteresis The perovskite structure remains stable at least to 170 GPa


3mR

At ambient pressure: spin-frustrated a.f. Hexagonal structure, very anisotropic

Fe3+ (S=5/2), Cu1+ (S=0)Finally at ~19 GPa a 3D super-exchange is realized. TN ~ 50 K

( )0

cd adP


0.90

0.95

1.00

-10 -5 0 5 10

0.90

0.95

1.00

6 K

Inte

nsity

AF Fe3+

AF Fe2+

120 K

AF Fe3+

0 20 400

100

200

300

Fe3+

Fe2+

TN (

K)

P (GPa)

296 K

260 K

27 GPa

PM Fe3+

Velocity (mm/s)

6 K

19 GPa

?1 3 2 20 5 2 1 2 2Cu Fe Cu FeS S S S

27 GPaCu2+

4 GPaCu1+


The rigidity of the OThe rigidity of the O2-2- – Cu – Cu1+1+ - O- O2-2-

dumbbell and its orientation along the dumbbell and its orientation along the cc-axis-axis

are responsible for the large anisotropy are responsible for the large anisotropy in delafossite.in delafossite.

With pressure increase the is With pressure increase the is doomed to collapse doomed to collapse

3mR


A series of:1 - PI structure transition 2 – Followed by PI electronic phase transition 3 - Which in turn leads to another structural phase transition

3 2/m C cR ®

1 3 2 20 5 2 1 2 2Cu Fe Cu FeP

S S S S

2/ 3C c P m®

3mR

2/C c

3mP

LP HP1 HP2

HP1 HP2

LP


We thus conclude a We thus conclude a serendipitousserendipitous voyage voyage into the extremities of matter.into the extremities of matter.

serendipity: the ability to make fortunate discoveries by accident

Pinta and Santa Maria

Discovery of America!

Discovery of fundamentals

of physics

DAC

Discovery of SC in HgKamerlingh-Ohnes (1911)

structural r espon s e to p ressure i nduced e lectronic t ransitions in tm-compounds

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