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Introduction to EXAFS II Examples

Introduction to EXAFS II 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 EXAFS; motivate each problem. Solid State lighting ; Cu dopants in ZnS

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Introduction to EXAFS II Examples

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  1. Introduction to EXAFS II 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 EXAFS; motivate each problem. • Solid State lighting ; Cu dopants in ZnS • Thermoelectric materials: distortions lead to electron and phonon scattering. Ba8Ga16Sn30 • Zn in LiNbO3, an optical crystal used in many applications. • Molecules Chalmers 2011

  2. AC Electroluminescence and solid state lighting • ZnS doped with Cu and Cl has the unusual property that it luminesces under AC voltage excitation (e.g. 100V across 50 µm) but does not luminesce using DC voltages. • Further the luminescence is not uniform but emanates from many tiny points < 1 µm. • Cu is not soluble in ZnS and nanoprecipitates of CuxS form for Cu concentrations of 0.15% Cu (75% precipitate, 25% Cu dopants). • CuxSnanoparticles are thought to be needle-like in nature and enhance the local E-field when the voltage is switched. • Local structure problems we addressed: • What is the CuxS structure CuS or Cu2S? • Can we determine the environment about isolated Cu atoms? Four ZnS:Cu,Cl particles (20-30 µm) under AC excitation; 100 V square wave. J. Phys.: Condens. Matter , 22, 055301 (2010); Phys. Rev. B , 75, 075301 (2007) Chalmers 2011

  3. First, can distinguish between different structures at local level - example for thin films Examples of CuxS structures Second peak (Cu-Cu) overlaps Cu-S peak for Cu2S (middle, green), but moves away from Cu-S peak as x decreases (blue and green in top) Example: Fit of thin film to the Cu-S and Cu-Cu peaks of Cu2S structure; verified that Cu2S was deposited. This sample was ~ 30 nm thick – nano-sized grains and no clear diffraction peaks observed. Chalmers 2011

  4. CuxS in ZnS is CuS-like to 2.6Å Substitutional Cu in ZnS has a very small shoulder in 2-2.5Å region, Cu2S has a large shoulder; CuS has a moderate shoulder. Fits to the CuS data with only tiny change in broadening and r. Structure beyond 4Å for ZnS:Cu suggests CuSepitaxially bonded to ZnS J. Phys.: Condens. Matter , 22, 055301 (2010); Phys. Rev. B , 75, 075301 (2007) Chalmers 2011

  5. CuS layer in the 111 plane of ZnS Zn S One c-axis unit cell plus one S-S double bond = 18.4Å Two 111 cube diagonals = 18.7Å; 1.6% mismatch Along interface, S-S distance in CuS is 3.80Å while in ZnS (111 plane) S-S distance = 3.825Å, only a 0.66% strain. CuS Phys D.: Appl. Phys., 44, 205402 (2011) Chalmers 2011

  6. Isolated Cu defect in ZnS • Problem: Cu nearly insoluble in ZnS – solubility limit near 0.04%. • Forms CuS precipitates: at 0.15% Cu, 75% of Cu in precipitates; only ~ 25% Cu are isolated defects. • Need very low concentrations to have only single defects. • Collaborators make Cu doped ZnSnanoparticles (NP). Optical fluorescence changes with added Cu. • Used low concentrations 0.02 and 0.04%; for these concentrations, and NP ~ 4 nm, < 1 Cu atom per NP on average – avoids clustering. • Observe same signal for many samples including up to ~ 0.4% Chalmers 2011

  7. 10K Fit of Cu data Cu K-edge; 0.02% Cu Cu K-edge; 0.02% Cu Cu K-edge; 0.04% Cu Zn K-edge FT range 3.5-10.5 Å-1 • Used experimental Cu-S standard function for first fits; R-space fit 1.4-2.2 Å. • Number of S neighbors ~ 3.2 and Cu-S distance short by 0.07-0.08Å. Cu K-edge; 4, 6 Cu-Cu Observe same Cu signal for many samples including up to ~ 0.4% Cu; Cu-Zn peak (12 nbrs) suppressed, Cu-S bond short Chalmers 2011

  8. New off-center Cu defect in ZnS:Cu • Only ~ 3.2 S neighbors – must be a S vacancy for many Cu atoms • Cu is Cu+ in ZnS; VS can compensate two Cu+ defects. • If Cu moves toward 3 S, moves away from vacancy; net displacement ~ 0.24Å from VS. • Off-center displacement splits Cu-Zn into three widely split peaks. • Constrained the Cu-Zn distances to the shortened Cu-S peak for a long r-space fit 1.2-3.8Å. Cu K-edge; 0.02% Cu NanoscaleDOI: 10.1039/c1nr10556f Chalmers 2011

  9. Background: Thermoelectric Materials • Can be used for heating or cooling. • Can generate electricity from waste heat (improving efficiency). • Good thermoelectric materials have high electrical conductivity and low thermal conductivity. • Clathrates are good because they have cage structures with rattler atoms that decrease thermal conductivity. Chalmers 2011

  10. Problem – low ZT for Ba8Ga16Sn30 • Ba8Ga16Sn30 has a lower thermal conductivity than Ba8Ga16Ge30, but a lower figure of merit. K. Suekuni. Phys Rev B 77, 235119 (2008) Chalmers 2011

  11. Distortions: Thermoelectric Properties • We expect additional disorder in BaGaSn since Ga and Sn are very different sizes whereas Ga and Ge are close. • Diffraction may miss this disorder due to non-uniform arrangement of atoms among the sites. Mixture of Ga and Sn on each crystallographic site. • We used EXAFS to probe individual elements. Chalmers 2011

  12. Ba8Ga16Sn30Sn K-edge EXAFS Results • Peaks are visible at ~2.3Å and ~2.6Å above for both n- and p-type. Actual fit distances are 2.65Å and 2.80Å. • FT window of 3-15.5Å, Gaussian broadened by σ=0.3. • Fit range of 1.9-2.7Å. Chalmers 2011

  13. Ba8Ga16Sn30Ga K-edge EXAFS Results • k- and r-space plots look like Ga-Sn and not Ga-Ga; mostly Ga-Sn bonds (85-90%). • Ga-Ga bonds short, and at ~same distance (2.55Å) as in Ba8Ga16Ge30. • σ2(T) for Ga-Sn pair is well described by correlated Debye model. • Little static distortion. Chalmers 2011

  14. Comparison of local distances in Ba8Ga16Sn30 with diffraction Blue-green M1 (6) Yellow M2 (16) Dark blue M3 (24) Most of the bonds are M2-M3 -- 48/92; next M1-M3 -- 24/92 • Large disorder of local bond lengths for Sn-Sn, Sn-Ga, & Ga-Ga will scatter both phonons and electrons. • Thus reduced κ does not lead to better ZT. Few Ga-Ga bonds; distance comparable to that in smaller unit cell for Ba8Ga16Ge30 Chalmers 2011

  15. Doped LiNbO3Optical crystals – birefringent, ferroelectric, photorefractive, 2nd harmonic generationDopants Zn, Mg, In, Fe etc what site do they occupy? Chalmers 2011

  16. Background: – change of properties of LiNbO3:Zn with dopant concentration. • Zn and other dopants in congruent LiNbO3 change the photo-refractive index , the second harmonic generation coefficient, lattice parameters etc. There is a critical concentration near 7% Zn where these parameters change more rapidly. • Congruent material • 1% Nb excess on Li site • 4% vacancies on Li site for compensation • Li0.95Nb1.01O3. • Li/(Li + Nb) = .485 What is the Zn substitution site (Li, Nb, or both) for higher Zn concentrations?? Volk etal, Appl. Phys. B, 72, 647 (2001) Chalmers 2011 Abdietal, Appl. Phys. B 68, 795 (1999)

  17. Background – substitution site • Many proposals for the substitution site – on the Li site (ZnLi); on Nb site (ZnNb); or on the interstitial site between Nb and Li along the c-axis. Some suggest a change in substitution site as concentration is increased above the critical concentration. Main models: ZnLi•+ VLi' or 3ZnLi•+ ZnNb' ' ' (others: 5ZnLi• + VNb ' ' ' ' '; 4 ZnNb' ' ' +3NbLi••••) Chalmers 2011

  18. Li site Large difference in amplitudes Zn-Li , Zn-Zn, Zn-Nb Nb site • Very large amplitude difference between Nb and Li neighbor at same distance. Can easily determine whether Nb neighbors are at 3.06 or 3.77Å; or on both . • Can tell if significant clustering of Zn occurs (above expected random distribution). • Amplitude Ratio: -- Zn-Nb:Zn-Zn:Zn-Li = 1:0.91:0.08 Chalmers 2011

  19. Zn K-edge data in LiNbO3:7.3 and 11.1% FFT: 4-14.2 Å-1 5% Zn 9% Zn • Largest peak is at 2.8 Å, where Zn on a Li site has a large Zn-Nb peak • Amplitude is low near 3.5 (where largest Zn-Nb peak would be for Zn on Nb site). • Little difference between the 5 and 9% data sets (similarly for 7%); main difference is a slightly lower amplitude for 9%, consistent with more disorder for 9% doping. Chalmers 2011

  20. Fits for Zn on Li site Fitting: Two Zn-O peaks (2.06, 2.26Å: N=3 each) Zn-Nb peak (3.06Å: N=4) Next Zn-O (3.28 +3.43Å: N = 6) Second Zn-Nb peak (3.36Å: N=3) Third Zn-Nb peak (3.87Å: N=1) Weak Zn-Li peak (3.77Å: N=6) Three long Zn-O (3.93-4.6Å) Two multi-scattering peaks Fit range 1.2-4.2Å FT range 4-14.2Å-1 Number of free parameters 21.5. Number varied 17; 1 amplitude, 6 distances, and 10 σ’s. Chalmers 2011

  21. Example of a fit for 11.1%Zn at 10K k-space 4-14.2 Å-1 General results: O peaks contract; first Zn-O bond contracts most -0.048 Å. Zn-Nb peak near 2.8 Å is very sharp and expands by +.068 Å; expansion of more distant Nb neighbors is less. Zn-Li (near 3.4Å) very weak – and parameters have large errors. Could be a tiny Zn-Zn peak for a Zn neighbor on Li at 3.77Å (0.66 neighbor with random distribution) ; no evidence for more than 1 Zn neighbor – no clustering Chalmers 2011

  22. Distortions about the ZnLi defect:an overall local expansion about Zn • Distances have little T-dependence • Closest Zn-O1 bond contracts • Closest Nb neighbor at 3.13Å is pushed away about 0.07Å; repulsion between Zn+2 and Nb+5. • Second (O2) neighbor almost unchanged; competition between attraction to Zn+2 and strong bond to Nb which is pushed away. • Further Nb shells slightly expanded – Zn substitutes on Li site. Lattice about Zn expanded – should increase lattice constants. Chalmers 2011

  23. Moleculesbrief examples Chalmers 2011

  24. XAFS can tell us whether uranium and phosphate form a complex U-Oax are very stiff cD~1000 K 4-5 other O neighbors EXAFS shows only one phosphide group present U-P much looser cD~300 K: XAFS of uranium-bacterial samples(CH Booth) One of several models

  25. Mn+2 partially changes from LS to HS [1,3-(Me3C)2C3H3]2Mn Low T: LS, First peak 10 Mn-C pairs (ring) 2.07-2.18A High T: HS, structure changes , more disordered, not reversible LS r = 2.14Å ; HS 2.43Å

  26. Example from Grant Bunker

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