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Introduction to EXAFS IV Examples

Introduction to EXAFS IV Examples. F. Bridges Physics Dept. UCSC, MC2 Chalmers. Scott Medling Michael Kozina Brad Car Yu (Justin) Jiang Lisa Downward C. Booth G. Bunker. Outline Examples using XANES Thermal vibrations Einstein modes Jahn –Teller Distortions Polarons

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Introduction to EXAFS IV Examples

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  1. Introduction to EXAFS IV Examples F. Bridges Physics Dept. UCSC, MC2 Chalmers Scott Medling Michael Kozina Brad Car Yu (Justin) Jiang Lisa Downward C. Booth G. Bunker Outline • Examples using XANES • Thermal vibrations Einstein modes • Jahn –Teller Distortions • Polarons • Off-center displacements Chalmers 2011

  2. 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. 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? 30 µm ZnS:Cu,Cl particle under 100V AC excitation. 0.15% Cu. Chalmers 2011

  3. Current model for AC ELZnS:Cu,Cl CuS has a different crystal structure – layered. • CuxS precipitate (NP); 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. ZnS Cl Cu Cu Cl Cu Cl CuxS Cu Cl Cu plays two roles, part of CuxS and isolated Cu is hole trap Fischer 1960’s Chalmers 2011

  4. AC EL degraded for small particles Milled 200 RPM, 2 min. 58 µm devices. 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!! 200 RPM 2 minutes, only slightly ground 400 PRM 2 minutes, many fine particles, but still some large particles Chalmers 2011

  5. EXAFS – size-selected particlesZnS: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! ZnS Cl Cu Cl CuxS Cl How can CuS precipitates, encapsulated inside ZnS, be damaged but ZnS is not? FFT ranges: Cu, 3.5-11.3Å-1 , Zn 4-13.5 Å-1, Mn 3.5-13.2 Å-1 Chalmers 2011

  6. 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? (CuSnano-precipitates embedded in the ZnS host.) Chalmers 2011

  7. 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. Chalmers 2011

  8. 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 Chalmers 2011

  9. He et al, Phys. Rev. B 76, 014401 (2007) Phase Diagram of La1-xSrxCoO3 • 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 localizedeg1 state it should be Jahn-Teller (JT) active; a JT distortion would be a signature for the IS spin state. 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) Chalmers 2011

  10. 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 intra-atomic 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? Chalmers 2011

  11. 10 Oe 30% Sr 1 T Bulk Magnetization1 μB = 5585 emu/mol 10%: Msat(5T) ~ 0.45 μB 15%: Msat(5T) ~ 0.78 μB Wu & Leighton, PRB 67, 174408 (2003) (powder samples) Chalmers 2011

  12. Phase Diagram 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 O Mn P. Schiffer et al. Phys. Rev. Lett. 75,3336(1995) La/Ca Pseudo-cubic ManganitesLa1-xCaxMnO3 (LCMO) • For x = 0.2 ~ 0.5, LCMO has a Colossal Magneto-Resistance (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? Chalmers 2011

  13. X-ray Absorption Near Edge Structure -comparison Edge shifts depend on bond lengths Manganites Cobaltites Manganites 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 PRB 63, 214405 (2001) Chalmers 2011 PRB 80, 144423 (2009)

  14. 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. 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 Chalmers 2011

  15. Co Pre-edge results, 10K 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. Chalmers 2011

  16. Compare σ2(T) for Co and Mn(Different distortions) Chalmers 2011

  17. 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. UCSC April 2011

  18. Non-phonon part of σ2(T) • 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. 30% Ca; B= 0 and 9T. 22% Ca, B=0 Complete JT/Polaron distortion Increasing JT/Polaron distortion. UCSC April 2011

  19. 2 vs T for x = 0.21, 0.3, 0.4, 0.45; Mn-O peak UCSC April 2011

  20. 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. UCSC April 2011

  21. 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. UCSC April 2011

  22. 4 K 300 K Co EXAFS r-space data for LSCO • 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. UCSC April 2011

  23. σ2(T) for bulk LSCO • σ2(T) follows the correlated Debye model very well; • Correlated Debye temperatures, θcD, are about 770 K ± 40K. • No indication of J-T distortion. UCSC April 2011

  24. Comparing LSCO with LCMO See PRL 102, 026401 (2009) • Comparison of σ2(T) for 20% bulk and nanoLa1-xSrxCoO3and 22% La1-xCaxMnO3: • 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. No significant JT distortion was found in LSCO (x = 0 – 0.35) at any temperature from 4 to 300 K. UCSC April 2011

  25. 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. Chalmers 2011

  26. CuO6Octahedra Hex. Axis C3v • If C3v symmetry retained, all axes equal. • No static Jahn-Teller distortion (i.e would require one of 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. O Cu Chalmers 2011

  27. 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? Chalmers 2011

  28. Ba3CuSb2O9 local structure different from average structure • EXAFS experiments on the 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. Cu-O Cu-Sb Cu K-edge data, 10K Chalmers 2011

  29. Combining EXAFS and Diffraction No Cu-Cu or Sb-Sb dumbbells. Cu-Sb dumbbells alternate in some way. Chalmers 2011

  30. 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. Chalmers 2011

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