Introduction to EXAFS II Examples

Introduction to EXAFS II Examples

F. Bridges Chalmers 2011

F. Bridges Physics Dept. UCSC,

MC2 Chalmers

Scott MedlingMichael KozinaBrad CarYu (Justin) JiangLisa 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

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).

• CuxS nanoparticles 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)


Examples of CuxS structuresSecond 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.

First, can distinguish between different structures at local level -

example for thin films

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 CuS epitaxially bonded to ZnS J. Phys.: Condens. Matter , 22,

055301 (2010); Phys. Rev. B , 75, 075301 (2007)

CuS layer in the 111 plane of ZnS


One c-axis unit cell plus one S-S double bond = 18.4ÅTwo 111 cube diagonals = 18.7Å; 1.6% mismatchAlong interface, S-S distance in CuS is 3.80Å while in ZnS (111 plane) S-S distance = 3.825Å, only a 0.66% strain.

Phys D.: Appl. Phys., 44, 205402 (2011)

Zn S

Cu S

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 ZnS nanoparticles (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%


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

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

• 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.

FT range 3.5-10.5 Å-1

10K

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.

Nanoscale DOI: 10.1039/c1nr10556f

Cu K-edge; 0.02% Cu

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.


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)

Sample ZTBaGaGe 1.35 @ 900K

BaGaSi 0.87 @ 870K

BaGaSn <0.6


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.


Ba8Ga16Sn30

Sn 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.


Ba8Ga16Sn30

Ga 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.

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

Few Ga-Ga bonds; distance comparable to that in smaller unit cell for Ba8Ga16Ge30

• 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.

Doped LiNbO3

Optical crystals – birefringent, ferroelectric, photorefractive, 2nd

harmonic generationDopants Zn, Mg, In, Fe etc what site do they occupy?


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.

Volk etal, Appl. Phys. B, 72, 647 (2001)

Abdi etal, Appl. Phys. B 68, 795 (1999)

• 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??


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••••)

Large difference in amplitudes Zn-Li , Zn-Zn, Zn-Nb


• 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

Li site

Nb site

Zn K-edge data in LiNbO3:

7.3 and 11.1%


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.

FFT: 4-14.2 Å-1

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 1.2-4.2ÅFT range 4-14.2Å-1

Number of free parameters 21.5.Number varied 17; 1 amplitude, 6 distances, and 10 σ’s.

Example of a fit for 11.1%Zn at 10K


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

k-space 4-14.2 Å-1

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

Moleculesbrief examples

F. Bridges

XAFS of uranium-bacterial samples(CH Booth)

0 1 2 3 4-15

-10

-5

0

5

10

15

U LIII edge

data at 298 K fit

FT o

f k3

(k)

r (Å)

Bacteria

• 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:

0 1 2 3 4-14-12-10-8-6-4-202468

101214

FT o

f k3

(k)

r (Å)

30 K 300 K

SalleiteU L

III edge

One of several models

Mn+2 partially changes from LS to HS

[1,3-(Me3C)2C3H3]2Mn

Low T: LS, First peak 10 Mn-C pairs (ring) 2.07-2.18AHigh T: HS, structure changes , more disordered, not reversible

LS r = 2.14Å ; HS 2.43Å

Example from Grant Bunker

Introduction to EXAFS II Examples

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