Structural r espon s e to p ressure i nduced e lectronic t ransitions in TM-compounds
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Transcript of Structural r espon s e to p ressure i nduced e lectronic t ransitions in TM-compounds
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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
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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….
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– 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
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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
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P
Fe3+
Fe2+
The high spin state is unstable at high-The high spin state is unstable at high-pressurepressure
P“Spin
crossover”
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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)
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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
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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
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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
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±±vvdetectordetector
Nuclear Nuclear resonant resonant scattererscatterer
Synchrotron Synchrotron monochromatimonochromati
c beamc beam
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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
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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
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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:
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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
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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
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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
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-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
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-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
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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
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T >> TN
Paramagnetic Fe2+
T << TN
anti-ferromagnetic
Fe2+
2 3Fe Fe
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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
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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!
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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+
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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
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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)
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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
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FeFe22OO33 a catastrophic correlation a catastrophic correlation
breakdownbreakdown
5/ 2S
0S
INSULATOR-METAL TRANSITION COLLAPSE OF MAGNETISM
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• 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
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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.
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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
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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
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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+
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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
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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
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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)