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DETERMINATION OF SHORT RANGE ORDER (SRO) PARAMETERS FROM EXAFS PAIR DISTRIBUTION FUNCTIONS OF OXIDE AND
SILICATE SOLID SOLUTIONS
G. Waychunas, W. Dollase, C. Ross
To cite this version:
G. Waychunas, W. Dollase, C. Ross. DETERMINATION OF SHORT RANGE ORDER (SRO) PARAMETERS FROM EXAFS PAIR DISTRIBUTION FUNCTIONS OF OXIDE AND SILI- CATE SOLID SOLUTIONS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-845-C8-848.
�10.1051/jphyscol:19868162�. �jpa-00226066�
JOURNAL DE PHYSIQUE
Colloque C8, suppl6ment au n o 12, Tome 47, dkcembre 1986
DETERMINATION OF SHORT RANGE ORDER (SRO) PARAMETERS FROM EXAFS PAIR DISTRIBUTION FUNCTIONS OF OXIDE AND SILICATE SOLID SOLUTIONS
G.A. WAYCHUNAS, W.A. DOLLASE* and C.R. ROSS'
Center for Materials Research, 105 McCullough Building, Stanford University, Stanford, CA 94305-4045, U.S.A.
" ~ e p a r t m e n t of Earth and Space Sciences, University of California at Los Angeles (UCLA), 405 Hilgard Avenue, Los Angeles, CA 90024, U.S.A.
A b s t r a c t - Short Range Order (SRO) i n c e r t a i n s o l i d s o l u t i o n s can be measured d i r e c t l y from observed EXAFS s t r u c t u r e f u n c t i o n peak areas. Results f o r MgO-FeO, MgO-Li FeO2, and CaMgSi O6 ~ a ~ e 2 + ~ i ~ 0 ~ s o l u t i o n s i n d i c a t e t h a t these a r e almost completely d i sordere5 a t t h e i r synthesis temperatures. The n e c e s s i t y o f s t r u c t u r a l modeling t o o b t a i n i n t e r a t o m i c distances f o r v a r y i n g compositions and degree of o r d e r i n g i s emphasized.
I INTRODUCTION
SRO parameters can be measured d i r e c t l y from EXAFS c o r r e l a t i o n f u n c t i o n s i n t h e case o f s o l i d s o l u t i o n s having f a v o r a b l e s t r u c t u r e s . A good example i s afforded by MgO-FeO s o l u t i o n s having t h e NaC1 s t r u c t u r e . The magnitude o f t h e second neighbor peak i n t h e d i s t r i b u t i o n f u n c t i o n f o r t h i s s o l u t i o n i s v e r y s e n s i t i v e t o t h e composition o f t h e second s h e l l . T h i s i s n o t o n l y t r u e because o f t h e markedly d i f f e r e n t b a c k s c a t t e r i n g amplitudes o f Fez+ and Mg, b u t i s a l s o due t o t h e l a r g e b a c k s c a t t e r i n g phase d i f f e r e n c e over t h e f u l l k range. C l u s t e r i n g o r
o r d e r i n g behavior would thus c r e a t e s u b s t a n t i a l d i f f e r e n c e s i n second peak area r e l a t i v e t o a random m i x t u r e o f c a t i o n s . Since t h e f o u r t h and s i x t h s h e l l s a r e a l s o composed o n l y o f c a t i o n s , SRO parameters i n v o l v i n g these s h e l l s a r e a l s o measurable i n p r i n c i p l e .
The determination o f SRO i n ceramic o r mineral s o l u t i o n s by standard X-ray methods i n v o l v i n g d i f f u s e i n t e n s i t y measurements i s a formidable task. The diffuse i n t e n s i t y i s r e l a t e d t o t h e square o f t h e d i f f e r e n c e i n t h e atomic s c a t t e r i n g f a c t o r s / I / . I n s u c c e s s f u l l y s t u d i e d metal systems, such as CuAu, t h i s value i s about 2500 e l e c t r o n u n i t s a t k=O. I n t h e MgO-FeO system t h e analogous v a l u e i s o n l y 196. The s i t u a t i o n i s even worse near t h e compositional l i m i t s , i.e. a t h i g h d i l u t i o n . However, EXAFS a n a l y s i s i s l a r g e l y independent o f element d i l u t i o n and good r e s u l t s should be o b t a i n a b l e even a t ppm l e v e l s o f t h e probe atom.
I n t h e present work we r e p o r t t h e measurement o f second neighbor SRO i n MgO-FeO (PW), MgO-LiFe02 (PLF) and c ~ M ~ s ~ ~ o ~ - c ~ F ~ ~ + s ~ ~ o ~ (DiHd) s o l i d s o l u t i o n s . The DiHd s o l u t i o n i s of g r e a t importance t o s t u d i e s o f subsolidus phase r e l a t i o n s i n t h e clinopyroxene (CPX) minerals. I n t h e CPX s t r u c t u r e chains o f SiO4 t e t r a h e d r a a r e h e l d together by an a l t e r n a t i n g chain o f s i x and e i g h t coordinated cations, i n t h e M1 and M2 s t r u c t u r a l s i t e s , r e s p e c t i v e l y . I n DiHd t h e l a r g e r M2 s i t e i s f i l l e d by Ca and t h e s m a l l e r s i t e by Mg o r Fez+. The M1 octahedra share edges forming a pseudo-one dimensional c h a i n through t h e s t r u c t u r e /2/. Each M1 octahedron a l s o shares edges w i t h t h r e e M2 polyhedra which do n o t share edges w i t h each other.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868162
C8-846 JOURNAL DE PHYSIQUE
Accordingly, EXAFS probe atoms l o c a t e d o n M1 s i t e s can sense t h e i d e n t i t y o f n e x t nearest neighbor atoms i n adjacent M1 and M2 s i t e s l e a d i n g t o d i r e c t determination o f MI-MI and MI-M2 SRO parameters.
I n cases of charge coupled s u b s t i t u t i o n s , as i n PLF and pyroxenes l i k e ~ a ~ e 3 + ~ i ~ 0 ~ - CaMgSi206 (Acmite-Diopside, AcDi) i t i s n o t known how c l o s e l y t h e charge compen-
s a t i n g species a r e located. The measurement o f SRO parameters should h e l p s o l v e t h i s long-standing problem i n s t r u c t u r a l chemistry.
I 1 SRO I N MgO-FeO SOLID SOLUTIONS
F i g u r e 1 shows a comparison o f t h e observed second s h e l l peak areas w i t h those c a l c u l a t e d from t h e o r y i t PW s o l u t i o n s assuming complete andomness. The samples were synthesized a t 1140 C and should have cimplete Mg-FeR d i s o r d e r /3/. The two t h e o r e t i c a l curves demonstrate t h e importance o f p r o p e r l y d e f i n i n g t h e i n t e r a t o m i c distances used i n t h e EXAFS refinement. The change i n i n t e r a t o m i c d i s t a n c e ( r ) i n PW can be modeled by d i s t a n c e l e a s t squares (DLS) techniques /4/. Such modeling p r e d i c t s near Vegard behavior o f r f g r second nearest neighbors ( b u t n o t a l l neighbors). Use o f r f i x e d a t 3.00 A s h i f t s t h e minimum i n t h e curve and has a poorer fit t o t h e observations. However, t h i s e f f e c t i s small i n PW s o l u t i o n s where t h e r e a r e no o t h e r types o f atoms c o n t r i b u t i n g t o t h e second s h e l l EXAFS, as w e l l as a l a r g e c o o r d i n a t i o n number (12) which gives a l a r g e t o t a l b a c k s c a t t e r i n g amplitude. I n more complex s o l u t i o n s such as DiHd, c o r r e c t modeling of t h e
distances, independent o f t h e EXAFS a n a l y s i s , i s e s s e n t i a l f o r q u a n t i t a t i v e r e s u l t s . I n t h e t h e o r e t i c a l c a l c u l a t i o n s we have used t h e b a c k s c a t t e r i n g amplitudes as compiled by Teo and Lee /5/, and phase s h i f t f u n c t i o n s obtained from model com- pounds. Use o f e m p i r i c a l amp1 i t u d e f u n c t i o n s would improve t h e f i t t o t h e observations, s i n c e t h e Teo and Lee Mg b a c k s c a t t e r i n g amplitudes appear t o be s y s t e m a t i c a l l y s m a l l e r than those t h a t we observe f o r second s h e l l s i n o t h e r model s t r u c t u r e s . F i g u r e 2 shows t h e r e f i n e d number o f second neighbor Mg atoms
determined from t h e EXAFS. The standard e r r o r i s about 0.5 atom. Work i s now c o n t i n u i n g on f o u r t h s h e l l SRO and a complete DLS p i c t u r e o f t h e s o l i d s o l u t i o n s t r u c t u r e .
2 8 26
2 4 2
2
2 0 18r DEFINED BY VEGARD 1 6
r CONSTANT = 3008.
1 4 1 2
0 10 20 30 40 50 60 70 80 90 100
NUMBER OF SECOND NEAREST NEIGHBORS IN MgO-FeO SOLID SOLUTIONS
MOLE % FeO MOLE 010 FeO
Fig.l 2nd s h e l l magnitudes i n P W s o l u t i o n s . Fig.2 2nd s h e l l Mg neighbors.
111 SRO I N MgO-LiFe07 SOLID SOLUTIONS
F i g u r e 3 shows t h e agreement between theory and observation f o r t h e charge coupled
MgO-LiFe02 s o l i d s o l u t i o n . A l l o f these samples were synthesized a t 1000
O C .The
t h e o r e t i c a l curve i s based on complete d i s o r d e r o f Mg, L i and Fe3+, and uses
Teo and Lee backscattering amplitudes, empirical phase s h i f t functions and Vegard trend interatomic distances. The observations a r e generally consistent with the theory. However, the 5% L F point f a l l s well below t h e theory curve. This may be due t o ~ e 3 + - v a c a n c ~ clustering in the sample which has been suggested by Mossbauer quadrupole s p l i t t i n g versus composition plots of the PLF solution /6/. Ordering may also be occurring i n the 100% L F sample which has been known t o have
considerable SRO a f t e r undergoing c e r t a i n annealing periods. Because of the complexity of possible simultaneous ordering and clustering in solutions such as PLF, additional data points need t o be collected before f u l l evaluation of SRO can be completed.
IV SRO in C ~ M ~ S ~ ,,oG-ca~e2+~i ?06 .SOLID SOLUTIONS
Pyroxene solutions a r e much.more d i f f i c u l t t o model than the NaCl s t r u c t u r e s due t o the l a r g e number of atoms necessary f o r a proper simulation. We are presently in the process of evolving DLS pyroxene models which incorporate variable SRO parameters f o r the description of the MI-M1 and MI-M2 ordering.
Figure 4 shows our f i r s t r e s u l t s using a Vegard trend t o estimate the interatomic distances f o r the theoretical (completely disordered) model. The agreement with observation i s good, except f o r t e lower composition Hd samples. This could be evidence f o r clustering of the Feqt i n t o MI s i t e s . However, a more l i k e l y p o s s i b i l i t y i s t h a t the theory predicts too small a second peak area f o r the low Hd compositions. This i s consistent with a s l i g h t l y low theoretical backscattering amplitude f o r Mg, as has been indicated i n the P W solution r e s u l t s .
The absence of a minimum in the rea versus composition curve i s due t o the r e l a t i v e l y small e f f e c t of ~ g - F e ? + mixing on t h e t o t a l second s h e l l peak area.
The area i s as much affected by the changing distances of the other second shell atoms, namely Si a t three d i f f e r e n t distances, Ca in M2 and 0. Because of t h i s the evaluation of SRO i n complex solid solutions must be done with great care i n order t o produce meaningful r e s u l t s . Modeling of the s t r u c t u r e i s probably t h e only way t o obtain sound interatomic distances f o r mu1 t i s h e l l EXAFS analysis.
Our DiHd samples were synthesized a t 800°C and 2 kbar. This temp rature i s s u f f i c i e n t t o remove much of the long range ordering of Mg and Felt in ortho- pyroxene /7/. However SRO information on any pyroxene system has never been measured by any method. Our work i s presently continuing on 3 other pyroxene CPX solutions and Omphaci t i c compositions.
Fig.3 2nd she1 1 magnitudes in MgO-Li Fe02 solutions .
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THEORY1 0
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1I . I , I
0 10 20 30 40 50 60 70 80 90 100 MOLE O h LF
JOURNAL DE PHYSIQUE
V CONCLUSIONS
i) In certain we1 1 defined sol id solutions SRO parameters can be measured directly from EXAFS structure function peak areas.
ii) Measurements and modeling on MgO-LiFeO2, MgO-FeO and Diopside-Hedenbergite solid solutions indicate that these are almost completely disordered at the temperatures used for sample preparation.
i i i ) The use of empirical phase shift functions is necessary in general for successful refinement of second or more distant shell EXAFS contributions.
iv) Crystal structure modeling is essential to obtaining proper interatomic distances for input to EXAFS second and more distant shell refinements.
Fig.4 2nd shell magnitudes DiHd solid solutions.
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7
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IN SECOND NEIGHBOR SHELL
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RDFPEAKAREA
3 1 I t I I I t I I I
0 10 20 30 40
