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EXAFS STUDY OF RELAXATION OF Cu+
IMPURITIES IN ALKALI HALIDES
T. Murata, S. Emura
To cite this version:
T. Murata, S. Emura. EXAFS STUDY OF RELAXATION OF Cu+ IMPURITIES IN ALKALI HALIDES. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-795-C8-798.
�10.1051/jphyscol:19868152�. �jpa-00226055�
EXAFS STUDY OF RELAXATION OF Cut IMPURITIES IN ALKALI HALIDES
T. MURATA and S. EMURA*
Department of Physics, Kyoto University of Education, Fushimi-ku, Kyoto 612, Japan
'Institute o f Scientific and Industrial Research, Osaka University-Suita, Campus, 8-1 Mihogaoka, Ibaraki, Osaka 565, Japan
-Abstract Cu K-edge EXAFS measurements have been done for NaCl:Cu+
in order to study the lattice relaxation around a cuprous ion. It was found that the bond length of Cu+ and C1- in NaCl takes almost the same value as that of CuCl crystal. This suggests a very large dis- tortion of the latice around a cuprous ion.
I . Introduction
TO study the lattice relaxation or the local lattice instabi- lity at an impurity ion site in a crystal, alkali halide crystals containing heavy metal ions of electronic configuration with dl0 type such as CU+ and ~ g + ions have been investigated bymeasurinqabsorptron spectra in ultraviolet region. Absorption bands were interpreted as the parity forbidden transition of the free ion2). In several examples, however, the oscillator strength is rather strong, and shows different tempera- ture dependence; little or n o dependence in KCl:Cu+ and a peculiar dependence in other systems like NaBr:Cuf, RbC1:Agf or RbBr:Ag+ which take a minimum value at a certain temperature. These phenomena are generally considered to be due to the situation where the substitu- tional CU+ ion is not located at a no a1 lattice site, but at an off- center position. Several experimentalSy and theoretical4) works have been done to determine the off-center distance and its direction. No definite results, however, have been obtained so far.
EXAFS is the most suitable method for observing the amount of the ionic relaxation in the lattice. As a first stage of the experimental investigation of the off-center relaxation problem, we chose a typical on-center system N ~ C ~ : C U + in order to check the applicability of EXAFS to the lattice relaxation problem.
2. Experimental
Single crystals NaCl:Cu+ were grown by Kyropoulos method in air.
Before melting, sample powder was hold at 400°C for one day in order to eliminate hydroxyl ions. The concentration of the CU+ ion in a grown crystal was found to be somewhat ten percent of the nominal do- ping value because of the low solubility of CU+ in NaC1. In some crys- tals the bottom part of the block looks slightly colored to yellow due to the aggregation of CuCl or a formation of micro-crystallite CuCl in a host. So we took this part as a sample of the aggregated CuCl cluster in NaCl host crystal (sample [a1 ). A transparent part of non-colored Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868152
C8-796 JOURNAL DE PHYSIQUE
c r y s t a l was t a k e n a s a w e l l d i s p e r s e d s a m p l e ( s a m p l e [ d l ) f o r t h e EXAFS measurement. The c o n c e n t r a t i o n s a r e e s t i m a t e d a s a b o u t 1 mol% f o r [ a ] from t h e amount of t h e e d g e jump of t h e x - r a y a b s o r p t i o n s p e c t r u m and a s l e s s t h a n 0 . 1 mol% f o r [dl from t h e r e s u l t s o f o p t i c a l measurements f o r t h i s c r y s t a l . CuCl was u s e d f o r a r e f e r e n c e m a t e r i a l .
F o r s a m p l e [ d l , a c r y s t a l b l o c k was a n n e a l e d f o r a b o u t 1 m i n u t e a t 7 0 0 ° C w h i c h i s a b o u t 5 0 ° C b e l o w t h e m e l t i n g p o i n t , t h e n r a p i d l y q u e n c h e d t o room t e m p e r a t u r e . The b l o c k w a s c l e a v e d a n d o n l y t h e i n n e r p a r t was c r u s h e d a n d m i l l e d i n a m o r t a r t o e n s u r e t o u s e t h e w e l l d i s p e r s e d p a r t , t h e n t h e p o w d e r was p r e s s e d t o a p e l l e t f o r m . T h i s p r o c e d u r e was p e r f o r m e d s h o r t l y b e f o r e e a c h EXAFS measurement i n o r d e r t o a v o i d t h e r e - a g g r e g a t i o n o f CuCl i n t h e h o s t . F o r s a m p l e s [ a ] and CuCl a p p r o p r i a t e amount o f e a c h s a m p l e was m i l l e d f i n e l y and mixed w i t h p o l y e t h y l e n e p o w d e r a n d p r e s s e d t o a p e l l e t f o r m i n o r d e r t o o b t a i n a s a m p l e w i t h u n i f o r m t h i c k n e s s .
EXAFS m e a s u r e m e n t s o n Cu K-edge w e r e c a r r i e d o u t a t EXAFS s t a - t i o n s i n s t a l l e d a t BL-1OB a n d a t BL-7C i n t h e P h o t o n F a c t o r y , t h e N a t i o n a l L a b o r a t o r y f o r High Energy P h y s i c s , KEK, Japan. Measurements w e r e c a r r i e d o u t a t room t e m p e r a t u r e e i t h e r i n a t r a n s m i s s i o n mode f o r CuCl a n d s a m p l e [ a ] o r i n a f l u o r e s c e n c e mode f o r s a m p l e [ d l The d e t e c t o r o f f l u o r e s c e n c e s i g n a l was t h a t d e v e l o p e d by L y t l e e t a 1 6 ) . 3. R e s u l t s and D i s c u s s i o n s
I n F i g s . 1 and 2 a r e shown t h e a b s o r p t i o n o r f l u o r e s c e n c e s p e c t r a and s p e c t r a o f m a g n i t u d e s o f F o u r i e r t r a n s f o r m o f t h e EXAFS f o r t h r e e s a m p l e s . Due t o t h e a b s e n c e o f o s c i l l a t i o n a t h i g h e n e r g y p a r t a n d r e l a t i v e l y l o w s i g n a l t o n o i s e r a t i o f o r e a c h s p e c t r u m , t h e u p p e r l i m i t s of wgndow f u n c t i o n f o r F o u r i e r t r a n s f o r m w e r e s e t o n l y t o , 5 0
9
eV ( k = 1 1 . 4 A - I ) f o r CuCl a n d f o r s a m p l e [ a ] , a n d 350 e V ( k = 9 . 6 A - ) f o r s a m p l e [ d l . The p o s i t i o n of a f i r s t s h e l l peak o f [ a ] i s e x a c t l y t h e same a s t h a t o f CuCl r e f e r e n c e , which shows t h e f o r m a t i o n o f CuCl m i c r o c r y s t a l l i t e c l u s t e r s i n t h e h o s t . An i n t e r e s t i n g p o i n t i s t h a t t h e h e i g h t o f t h e F o u r i e r p e a k o f [ a 1 i s a b o u t 1.5 t i m e s l a r g e r t h a n t h a t o f CuC1. T h i s may b e d u e t o t h e d e c r e a s e o f D e b y e - W a l l e r t y p e p a r a m e t e r a i n t h e c a s e o f c l u s t e r i n g , w h i c h m e a n s t h e r i g i d n e s s o f CuCl c l u s t e r i n t h e h o s t i n c r e a s e s . A s i m i l a r phenomenon was a l r e a d y o b s e r v e d i n t h e c a s e of KBr i n K C 1 m a t r i x 7 ) .
The F o u r i e r peak of [ d l a l s o a p p e a r s a l m o s t a t t h e same p o s J t i o n a s i n CuCl a n d [ a ] . I n a d d i t i o n , a s m a l l p e a k a t a b o u t 2.70 A c a n a l s o b e o b s e r v e d . I t i s r a t h e r s t r i k i n g t h a t t h e f i r s t s h e l l d i s t a n c e of Cu+ i n d i l u t e s a m p l e i s n o t a t t h e d i s t a n c e o f NaCJ h o s t , b u t t a k e s t h e v a l u e c l o s e t o p u r e CuC1. The p e a k a r o u n d 2.70 A c o r r e s p o n d s t o t h e d i s t a n c e between ~ a + and C 1 - i o n i n t h e h o s t , which was a n a l y z e d from a n EXAFS s p e c t r u m o f Na K-edge i n ~ a ~ 1 ~ ) . The n e i g h b o r i n g c h l o r i n e i o n s seem t o b e a t t w o d i f f e r e n t s i t e , t h a t i s , a t a d i s t a n c e s i m i l a r t o t h e v a l u e o f CuCl and a t t h a t o f NaC1. T h i s means t h a t t h e l a t t i c e d e f o r m s t o q u i t e a l a r g e e x t e n t a r o u n d a c u p r o u s i o n i m p u r i t y . A s i m i l a r p h e n o m e n o n w a s o b s e r v e d i n t h e c a s e o f s o l i d s o l u t i o n s l i k e G ~ A S - I ~ A S ' ) o r K C I - K B ~ ~ ) .
I n t h e c a s e o f d i l u t e m i x t u r e l i t h a s g e n e r a l l y b e e n a s s u m e d t h a t a Cu+ i o n i s s u b s t i t u t e d w i t h a ~ a i o n a n d t h e f i r s t n e i g h b o r d i s - + t a n c e o f Cu+ t a k e s t h e same v a l u e of t h e h o s t m a t r i x . The r e s u l t o f EXAFS i s c o n t r a r y t o t h e a b o v e p o s t u l a t e . T h i s f a c t m e a n s t h a t t h e l a t t i c e r e l a x a t i o n p r o b l e m a r o u n d a n i m p u r i t y i o n m u s t b e r e c o n s i d e r e d from t h e p o i n t o f v i e w o f s h o r t r a n g e o r d e r .
PHOTON ENERGY
/ k e VRRDIRL DISTRNCE R/R
Fig. 1 Copper K-edge extended Fig. 2 Fourier spectra for absorption spectra for CuC1, each sample. The difference aggregated CuCl in NaC1, and in the ordinate scale of the dispersed Cu' in NaC1. The last figure from others is last spectrum was taken in a due to the Fourier transform fluorescence mode. from the unnormalized fluo-
rescence EXAFS spectrum.
-Acknowled ements The authors are indebted to Drs. M. Nomura and A.
K-%d-P=on Factory for their kind assistance for the experl- ment. They would like to thank Dr. H. Maeda for the collboration in the EXAFS measurements. They are also grateful to Dr. F. W. Lytle for his kindness of loaning his fluorescence detector.
References
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