Download Chalmers 2011

Introduction to EXAFS IV
Examples
F. Bridges
Physics Dept. UCSC,
MC2 Chalmers
Outline
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Examples using XANES
Thermal vibrations Einstein modes
Jahn –Teller Distortions
Polarons
Off-center displacements
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Scott Medling
Michael Kozina
Brad Car
Yu (Justin) Jiang
Lisa Downward
C. Booth
G. Bunker
Chalmers 2011
More AC Electroluminescence:
low emission for small ZnS:Cu,Cl particles
Emission from tips of needle-like CuS precipitates
• For 20-30 µm particles, devices are ~ 50 µm
thick, and require ~ 100 V AC voltage.
• If can reduce operating voltage to 10V, easier
AC EL operation and 10 times less power.
• Smaller particles should allow 5-10 µm
devices.
30 µm ZnS:Cu,Cl particle
under 100V AC excitation.
0.15% Cu.
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Problem:
Slightly smaller particles do allow
operation at lower voltages
BUT!– small particles have very low
AC EL, even in 50 µm thick device.
Why?
Chalmers 2011
Current model for AC EL
ZnS:Cu,Cl
CuS has a different crystal structure – layered.
• CuxS precipitate (NP);
ZnS
Cu Cl
Cl
Cu
CuxS
Cl
Cu
Cu
Cl
conductor in
an insulating host. Small – nm scale.
• Isolated Cu – hole traps
• Cl atoms -- shallow electron traps.
• During field switching, large fields at
sharp tips – injects holes and
electrons on opposite ends of NP.
• Injected electrons/ holes trapped;
next time field is switched, then
electron-hole recombination occurs
emitting light.
Cu plays two roles, part of CuxS
and isolated Cu is hole trap
Fischer 1960’s
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Chalmers 2011
AC EL degraded for small
particles
Milled 200 RPM, 2 min.
58 µm devices.
200 RPM 2 minutes, only slightly
ground
400 PRM 2 minutes, many fine
particles, but still some large particles
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25 µm device works at lower V
To explore particle size effect, size-separated
particles : < 3 µm; 3-15 µm; 15-30 µm.
3 µm particles – very little ACEL!!
Chalmers 2011
EXAFS – size-selected particles
ZnS:Cu,Mn,Cl
Early EXAFS on ground particles showed some
decrease in amplitude but varied from sample to
sample.
• Using size-selected particles –
immediate result.
• The smaller the particles the
more the Cu environment was
damaged. (Cu K-edge)
• Surprisingly the Zn
environment - same sample showed no evidence of damage
• Even more surprising, another
dopant, Mn, also showed no
damage!
How can CuS
precipitates, encapsulated
inside ZnS, be damaged
but ZnS is not?
ZnS
FFT ranges: Cu, 3.5-11.3Å-1 , Zn 4-13.5 Å-1, Mn 3.5-13.2 Å-1
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Chalmers 2011
C
Cu
l
C
C
Cl
XANES -- similar results
Cu K-edges shows very large change – the
large edge structure observed for large
particles decreased as particle size
decreased → suggests disorder on few unit
cell level.
Zn and Mn K-edges show no indication of
any change – traces for the three sizes
overlap nearly perfectly.
How?
(CuS nano-precipitates embedded in the
ZnS host.)
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Chalmers 2011
ZnS:Cu,Mn,Cl fractures
along 111 planes
• Experiments on single crystals suggested that needle-like CuS precipitates
form along 111 planes; EXAFS suggests epitaxial growth - but small strain in
111 plane.
• Zinc Blende materials usually cleave on 111 planes, may do so more easily
along a strained plane.
• The doped ZnS particles also etch on 111 planes.
• If ZnS:Cu preferentially cleaves through the
CuS precipitates, the CuS NP will end up on
surfaces of smaller particles.
• Grinding always damages the surface, but for
host ZnS a negligible fraction of material ,
even for 0.1 µm particles.
• For the CuS NP , most of it may end up on
surface for small particles and be damaged.
• Small particles made via grinding not viable
for AC EL.
F. Bridges
Chalmers 2011
La1-xSrxCoO3
An unusual magnetic system. Formal Co valence increases with Sr doping.
For LaMnO3 no Co moment at low T but moment develops near 100K –
called a spin state transition.
With Sr doping becomes ferromagnetic and above 18% Sr, metallic
Early neutron PDF analysis suggested a significant JT distortion present
when moment present.
La1-xCaxMnO3
Colossal magnetoresistance material - large JT distortion for Mn3+ .
Formal Mn valence increases with Ca doping
Ferromagnetic metal for ~ 20-50% Ca,
Compare these two materials
F. Bridges
Chalmers 2011
Phase Diagram of
La1-xSrxCoO3
(a) The magnetic phase diagram in zero-field. At
very low doping, the system is nonmagnetic; it
slowly transforms into a spin glass (SG), then to a
spin-cluster-glass phase, and then becomes
ferromagnetic (FM).
(b) The magnetic susceptibility of LaCoO3. The
transition observed near 100 K is due to the
thermal activation of some diamagnetic low-spin
Co atoms (LS, t2g6eg0) to an intermediate-spin
(IS, t2g5eg1) or high spin (HS, t2g4eg2) state
configuration .
(c) If there is a localized eg1 state it should be JahnTeller (JT) active; a JT distortion would be a
signature for the IS spin state.
He et al, Phys. Rev. B 76, 014401 (2007)
LaCoO3
Major Question: Is there a J-T splitting (and IS
state)?
Is there a difference between bulk and nano?
D. Louca et. al., Phys. Rev. B 60, 10378 (1999)
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Chalmers 2011
LS-IS-HS of Localized Trivalent Co
O. Toulemonde et. al. Journal of Solid State Chemistry 158, 208-217 (2001)
Schematic diagram of the spin states for trivalent Co ions:
(A) low spin, LS (t62g) with ΔCEF>Jex, where Jex is the intraatomic exchange energy and ΔCEF is the crystal field energy;
(B) intermediate spin, IS (t52ge1g) with Jahn-Teller distortion;
(C) high spin, HS (t42ge2g ) with ΔCEF<Jex.
With Sr doping, are electrons removed from t2g or eg bands,
or do holes go into O bands?
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Chalmers 2011
10 Oe
30% Sr
1T
10%: Msat(5T) ~ 0.45 μB
15%: Msat(5T) ~ 0.78 μB
Wu & Leighton, PRB 67, 174408 (2003)
(powder samples)
Bulk Magnetization
1 μB = 5585 emu/mol
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Chalmers 2011
Pseudo-cubic Manganites
La1-xCaxMnO3 (LCMO)
Phase Diagram
O
Mn
La/Ca
6 equal Mn-O in CaMnO3 at ~1.9 Å
•
Mn-O are distorted in LaMnO3
2 Mn-O at ~1.90 Å
•
2 Mn-O at ~1.97 Å
2 Mn-O at ~2.15 Å
P. Schiffer et al. Phys. Rev. Lett. 75, 3336(1995)
For x = 0.2 ~ 0.5, LCMO has a Colossal MagnetoResistance (CMR).
In this concentration range, FM metal at low T – no JT
distortion; paramagnetic insulator at high T – large JT
distortion. What is in between?
Question: What is the distortion associated with the JT polaron?
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Chalmers 2011
1.8
normalized data
7-knots spline
X-ray Absorption Near Edge Structure -comparison
1.2
0.9
0.6
7800
8000 8200
E (eV)
8400
Manganites
Edge shifts depend on
bond lengths
Manganites
1.5
Large shift with Ca
concentration
Edge shape is nearly
same –i.e. a nearly
rigid edge shift –
about 3 eV per
valence unit.
Cobaltites
Essentially no edge
shift with Sr
concentration (bulk
or nano material).
Shift is less than
0.15eV
1.8
1.2
Cobaltites
0.9
0.6
0.3
0.15
0.10
0.05
7710
7715
E (eV)
7720
0.0
7700 7710 7720 7730 7740 7750 7760
E (eV)
1.8
Nano LSCO
15%
1.5
20%
25%
1.2
30%
0.20
0.9
35%
0.6
0.3
0.15
0.10
0.05
0.00
7705
7710
7715
E (eV)
7720
0.0
7700 7710 7720 7730 7740 7750 7760
E (eV)
PRB 80, 144423 (2009)
Chalmers 2011
0.20
0.00
7705
PRB 63, 214405 (2001)
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Bulk LSCO
0%
15%
20%
30%
Absorption (a.u.)
7600
Absorption (a.u.)
b)
0.0
Absorption (a.u.)
0.3
Absorption (a.u.)
Absorption (a.u.)
1.5
Co XANES – summary
• No significant Co K-edge shift for La1-xSrxCoO3 up to x = 0.35,
very few Co4+ (d5) are present
• in other Co systems the edge shifts ~3 eV/valence unit, and up to Co3.5
in NayCoO2; (Poltavets etal PRB 74, 125103 (2006)).
• The Co-O bond length is nearly constant with increasing Sr
concentration – in both diffraction and EXAFS. No valence change
from Bond-valence model.
V  6e ( r0  rC oO ) / B
drCo O
dV

V
B
Bond-valence model – valence (V) is directly dependent
on sum over nearest neighbor bond lengths. A lack of
bond length change as the Sr concentration increases
also indicates no Co valence change (B~0.37Å).
Conclusion:
The holes must be introduced primarily into the O(2p) bands
O K-edge XANES also suggest holes on O
F. Bridges
Chalmers 2011
1.8
1.2
0.9
Nano LSCO
15%
20%
25%
30%
35%
0.6
0.3
0.20
Absorption (a.u.)
Co Pre-edge results, 10K
Absorption (a.u.)
1.5
0.15
0.10
0.05
0.00
7705
7710
7715
E (eV)
7720
0.0
7700 7710 7720 7730 7740 7750 7760
E (eV)
First peak recently shown to be a quadrupole transition to the eg state. Second pre-edge peak
transition is to hybridized eg-4p state – amplitude determined by hybridization. (Vanko etal,
arXiv:0802.2744v1).
When (bulk) sample is magnetized (increasing x), peaks move together; for nanoparticles
(weakly magnetic), peaks do not shift with x.
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Chalmers 2011
2
σ (T)
Compare
for
Co and Mn
(Different distortions)
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Chalmers 2011
Mn EXAFS:- T and B dependence
30% Ca, B=0T; T dependence.
30% Ca, T = 270K; B dependence
We fit the first few peaks to a sum of EXAFS
functions (from FEFF), with Gaussian PDFs and
focus on the first Mn-O peak. The disorder is
parameterized by σ, the PDF width.
F. Bridges
UCSC April 2011
Non-phonon part of
σ2(T)
30% Ca; B= 0 and 9T.
• σ2(T) vs T for ordered CaMnO3 – phonons only
(solid line).
• Parallel to high T data (above Tc) for all samples.
• Subtract σ2 phonons(T) from data for CMR samples
–flat above Tc
• The polaron correlations just above Tc do not
change the local distortions.
22% Ca, B=0
6
Complete JT/Polaron distortion
2
-3
2
JT/polaron (10 Å )
5
4
Increasing JT/Polaron
distortion.
3
2
1
0
x=22%
0
100
200
300
T (K)
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UCSC April 2011
400
500
600
2 vs T for x = 0.21, 0.3, 0.4, 0.45;
Mn-O peak
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UCSC April 2011
2JT/polaron vs M/MT; B  2T
• Magnetization does not saturate
until B  0.4T at low T (30%
Ca); requires higher B field for
other samples, other
temperatures.
• For B fields  2T; a universal
curve for given sample.
• Curve provides a direct
relationship between local
distortion and magnetization.
• Small amount of distortion
removed until the sample is
roughly 60% magnetized. Slope
is the same for every sample.
• Above 60%, decrease in
disorder per Mn site is faster
and slope has largest magnitude
for lower concentrations.
F. Bridges
UCSC April 2011
Dimeron Polaron Model
• For Dimeron, EJT small and comparable on 2 sites; electron
(hole) shared over 2 sites -- these are the low distortion sites.
• Dimeron confined by large JT distortions on neighboring
Mn atoms.
• extended electron state – or very fast-hopping; on core-hole
lifetime scale??
• Double exchange operative – spins on each site parallel -spin 7 object. Will have some weight on central O atom.
Low Ca conc.
Transport via activated
hopping – energy
barrier ~½ EJT.
F. Bridges
UCSC April 2011
Co EXAFS r-space data for LSCO
0.6
0.6
20% bulk LSCO(MZ)
0.4
0.4
0.2
0.2
FFT ( k(k) )
FFT ( k(k) )
20% bulk LSCO(SA)
0.0
-0.2
-0.4
-0.6
0.0
300 K
-0.2
-0.4
0
2
4
r (Å)
6
8
0.6
20% nano LSCO(SA)
0.4
FFT ( k(k) )
4K
-0.6
-0.2
-0.4
-0.6
0
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2
4
r (Å)
6
2
4
r (Å)
6
8
•
There is little difference between bulk samples
of the same Sr concentration made by two
collaborators.
•
The nano samples with the same concentration
have smaller amplitudes for the longer Co-X
peaks; the peak amplitudes decreases more
rapidly with distance.
0.2
0.0
0
8
UCSC April 2011
σ2(T)
x = 0.15 bulk
Data (Sundaram Sample - Run 2)
Fit (cD= 813 K)
x=0
5.0
Data (Mitchell Sample - Run 2)
Fit (cD= 762 K)
4.5
4.0
3.5
3.0
b)
a)
2.5
5.0
2
-3
2
 (10 Å )
for
bulk LSCO
5.5
4.5
x = 0.2 bulk
Data(Mitchell Sample - Run 1)
Fit (cD= 782 K)
Data(Sundaram Sample - Run 2)
Fit (cD= 763 K)
x = 0.3 bulk
Data(Mitchell Sample - Run 1)
Fit (cD= 733 K)
Data(Mitchell Sample - Run 2)
Fit (cD= 756 K)
4.0
3.5
3.0
2.5
0
100
200
300 0
T (K)
•
•
•
σ2(T) follows the correlated Debye model very well;
Correlated Debye temperatures, θcD, are about 770 K ± 40K.
No indication of J-T distortion.
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d)
c)
UCSC April 2011
100
200
300
Comparing LSCO with LCMO
Comparison of σ2(T) for 20% bulk and nano
La1-xSrxCoO3 and 22% La1-xCaxMnO3:
See PRL 102, 026401 (2009)
10.0
20% bulk LSCO
Fit (cD= 763 K)
20% nano LSCO
Fit (cD= 826 K)
22% LCMO
8.0
-3
7.0
2
2
 (10 Å )
9.0
6.0
•
•
5.0
The LCMO sample has a large Jahn-Teller distortion
that develops between 100 and 190 K.
All the LSCO samples, either bulk or nano, show
similar σ2(T) as the 20% bulk LSCO – fits correlated
Debye model with very little static distortion.
4.0
3.0
0
100
200
T (K)
300
No significant JT distortion was found in
LSCO (x = 0 – 0.35) at any temperature
from 4 to 300 K.
F. Bridges
UCSC April 2011
Ba3CuSb2O9 an unusual
magnetic system
•
Cu should be Cu+2 in this material and therefore Jahn-Teller (JT) active (d9).
•
Cu+2 is a spin ½ atom; if there is a crystal field splitting from a significant JT interaction
spin interactions should be anisotropic – i.e. g-factor in ESR should be anisotropic.
•
However in most samples g-factor appears isotropic and sample retains C3v symmetry
about the hexagonal axis. Appears inconsistent with a JT active atom or indicates that the
JT distortion is small! – or?
•
One of samples does show an orthorhombic distortion in neutron scattering – and an
anisotropic g-factor - more as expected, but why only one of many samples?
•
Problem for diffraction in orthorhombic space group cmcm : Cu and one Sb share a
crystallographic site and form Cu-Sb pairs. In diffraction the Cu and Sb occupations are
random – but in EXAFS strongly correlated; always Cu-Sb, and never Cu-Cu or Sb-Sb.
F. Bridges
Chalmers 2011
Hex. Axis
C3v
CuO6 Octahedra
• If C3v symmetry retained, all axes equal.
• No static Jahn-Teller distortion (i.e would require one of
O
Cu
F. Bridges
axes to lengthen).
• For JT distortion, the Cu-O distribution would split into
two long and four short Cu-O bond.
• JT theorem does not require static distortion – may be
dynamic. Then time scale becomes important.
• On ESR time scale (10-10 s) C3v symmetry retained on
average.
• On neutron time scale (10-12 s) C3v symmetry retained at
300K in Ortho sample.
Chalmers 2011
Possible structures from neutron
diffraction
Average Hexagonal structure
P63/mmc
Are Cu/Sb(2) randomly occupied or
correlated ?
Is the octahedron about Cu symmetric
or JT distorted?
F. Bridges
Chalmers 2011
Ba3CuSb2O9 local structure
different from average structure
• EXAFS experiments on the
•
•
•
Cu-O
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Cu-Sb
Cu K-edge data, 10K
Chalmers 2011
Orthorhombic and Hexagonal samples
is identical (blue dots, red squares)
Cu-O peak strongly suppressed in
EXAFS data – consistent with
significant Jahn-Teller distortion for
both samples.
Surprisingly , the small Cu-Sb peak
(one neighbor) is large in EXAFS but
smaller and at a shorter distance in
the calculated structure (for any
structure - P63mc, P63/mmc, or cmcm)
Large differences between EXAFS
peak positions experimentally and
those calculated from structure
determined in diffraction.
Combining EXAFS and Diffraction
No Cu-Cu or Sb-Sb dumbbells.
Cu-Sb dumbbells alternate in
some way.
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Chalmers 2011
Ba3CuSb2O9 - fit results
EXAFS: Cu-O peak is split - large JT splitting - and identical in both samples
in EXAFS, and almost unchanged from 10 to 300K.
Cu-Sb peak is very well ordered (small σ2 at low T) and significantly longer
(0.07Å) compared to diffraction.
How can the ESR data show no JT splitting in most samples and the neutron
scattering data only shows a large JT distortion at low T?
Answer: must consider time scales: ESR 10-9 - 10-10 sec; neutron scattering
10-12 sec; phonons 10-13 sec; EXAFS 10-15 sec. The JT distortions are fluctuating
rapidly, generally much faster that ESR time scales and comparable or faster
than neutron scattering time scales. Fluctuations are slow on EXAFS time
scales and the full JT distortion is observed.
For magnetism the spin interaction with the structure is motionally averaged!
Thus can consider the spin ½ interactions without structural interaction.
F. Bridges
Chalmers 2011