11.09 11.1 11.11 11.12 11.13 g-GeO 2 recovered q-GeO 2 recovered q-GeO 2 pure GeO 2 glass rutile GeO 2 g-GeO 2 q-GeO 2 14GPa g-GeO 2 14GPa Photon Energy ( keV ) Absorption ( arb. unit) F i g . 1 . E x p e r i m e n t a l G e K - e d g e X A N E S s p e c t r a o f v a r i o u s G e O 2 s a m p l e s . A r r o w s i n d i c a t e t h e t w o shoulders characteristic of six-fold coordination of Ge. XAFS STUDY OF GeO 2 UNDER PRESSURE When quartz-type crystalline GeO 2 ( q-GeO 2 ) is c o m p r e s s e d t o p r e s s u r e s o f 1 0 G P a a t t e m p e r a t u r e s c a p a b l e o f i n h i b i t i n g t h e c r y s t a l l i z a t i o n o f t h e r m o d y n a m i c a l l y s t a b l e h i g h - p r e s s u r e p h a s e s , a p r e s s u r e - i n d u c e d p h a s e tr an si ti on oc cu rs al te ri ng th e co or di na ti on nu mb er ( C N ) o f G e f r o m 4 t o 6 [ 1 ] . S o m e g r o u p s h a v e reported the high-pressure phase to be amorphous, while others insist the phase is crystalline in nature [2] . The sixfold coordination of Ge is either partly [1] or completely [2] preserved after the release of p r e s s u r e . P r e s s u r e - i n d u c e d C N c h a n g e s h a v e a l s o b e e n o b s e r v e d f o r v i t r e o u s G e O 2 a t 6 - 1 0 G P a , a l t h o u g h t h e s e c h a n g e s a r e r e v e r s i b l e [ 1 ] . Alkali germanate glasses, prepared under ambient c o n d i t i o n s , c o n t a i n a m a x i m u m o f 2 0 % o f t h e si xf ol d co or di na te d Ge , th ro ug ho ut a co mp os it io n r a n g e f r o m 1 0 t o 3 0 m o l % a l k a l i o x i d e . T h i s phenomenon is known as germanate anomaly [3] . T h e p r e s s u r e g e n e r a t e d w a s c a l c u l a t e d from the equation of state of Au u s i n g A u - A u d i s t a n c e s d e t e r m i n e d b y A u L - e d g e E X A F S m e a s u r e m e n t s . M e a s u r e m e n t s were made at room temperature. EXAFS d a t a a n a l y s i s i s d e s c r i b e d i n d e t a i l elsewhere [5] . T o d i s t i n g u i s h b e t w e e n f o u r f o l d a n d s i x f o l d c o o r d i n a t i o n , w e r e c o r d e d t h e X A N E S s p e c t r a o f q - G e O 2 a n d g - G e O 2 u n d e r v a r i o u s c o n d i t i o n s ( F i g . 1 ) . T h e X A N E S s p e c t r a d i s p l a y t w o d i s c e r n i b l e s h o u l d e r s , c h a r a c t e r i s t i c o f a s i x f o l d c o o r d i n a t i o n o f G e . A t 1 4 G P a , q - G e O 2 d e m o n s t r a t e s t h e s h o u l d e r s . T h e s e f e a t u r e s , h o w e v e r , d i s a p p e a r w h e n q - G e O 2 i s r e s t o r e d t o a m b i e n t p r e s s u r e , i n d i c a t i n g a n a l m o s t c o m p l e t e r e v e r s e t r a n s i t i o n i n t h e q - G e O 2 s a m p l e . T h e s p e c t r a o f s t a r t i n g g - G e O 2 g l a s s a l r e a d y We have compressed quartz-type crystalline GeO 2 ( q - G e O 2 ) a n d v i t r e o u s L i 2 O - 4 G e O 2 ( g - G e O 2 ) , a t r o o m t e m p e r a t u r e t o p r e s s u r e s u p t o 1 4 G P a t o ob se rv e lo ca l st ru ct ur al ch an ge s by in -s it u XA FS ; this method allowed the investigation of the nature of the pressure-induced transition [4] . q- Ge O an d g- Ge O we re we ll -g ro un d, mi xe d w i t h a m o r p h o u s b o r o n , a n d p l a c e d i n a h i g h - pressure cell made of boron and epoxy resin. Au foil, serving as a pressure marker, was separately charged in the cell. A cubic-type multi-anvil press ( S M A P 1 8 0 ) , i n s t a l l e d o n t h e B L 1 4 B 1 b e n d i n g magnet beamline, was used for compression. The X - r a y b e a m , m o n o c h r o m a t i z e d b y S i ( 1 1 1 ) r e f l e c t i o n , w a s f o c u s e d v e r t i c a l l y b y t w o m i r r o r s . T h e i n c i d e n t X - r a y b e a m w a s d i r e c t e d i n t o t h e sample using vertical (0.2 mm ) and horizontal (0.3 mm ) slits. We then measured the X-ray absorption s p e c t r a n e a r t h e G e K - e d g e a n d t h e A u L - e d g e . 2 2 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 0 5 10 15 Pressure ( GPa ) σ σ 2 (Å 2 ) CN = 4 CN = 6 q-GeO 2 comp q-GeO 2 decomp g-GeO 2 comp g-GeO 2 decomp Fig. 2. Variation of Ge-O distance of quartz-type q-GeO 2 a n d g - G e O 2 b y c o m p r e s s i o n ( s o l i d s y m b o l s ) a n d d e c o m p r e s s i o n ( o p e n s y m b o l s ) . C i r c l e s s i g n i f y q - G e O 2 while the squares indicate g-GeO 2 . Solid lines provide a guide for the eye only. Fig. 3. Variation of Debye-Waller factors, σ 2 of q-GeO 2 a n d g - G e O 2 , b y c o m p r e s s i o n ( s o l i d s y m b o l s ) a n d d e c o m p r e s s i o n ( o p e n s y m b o l s ) . C i r c l e s i n d i c a t e q - G e O 2 and the squa res desig nate g-Ge O 2 . Solid lines prov ide a guide for the eye only. di sp la ys sl ig ht tr ac es of th e sh ou ld er s, demonstrating the germanate anomaly. A t 1 4 G P a , t h e s p e c t r a p o s s e s s t h e c h a r a c t e r i s t i c s o f s i x f o l d G e c o o r d i n a t i o n . T h e s p e c t r a o f g - G e O 2 a l l o w e d t o re co ve r fr om hi gh pr es su re ar e si mi la r to those of the starting glass. Co mp re ss io n an d de co mp re ss io n g r e a t l y v a r i e s t h e G e - O d i s t a n c e s o f b o t h q - G e O 2 a n d g - G e O 2 ( F i g . 2 ) . Throughout the compression of q-GeO 2 b e l o w 8 G P a , t h e G e - O d i s t a n c e g r a d u a l l y s h o r t e n s , i n d i c a t i n g t h e c o n v e n t i o n a l c o m p r e s s i o n p a t t e r n o f q u a r t z - t y p e c r y s t a l l i n e s t r u c t u r e s . A n a b r u p t i n c r e a s e i n G e - O d i s t a n c e , h o w e v e r , o c c u r s b e t w e e n 8 a n d 1 2 GPa, corresponding to the CN change. The extrapolation of Ge-O distances at pressures ranging from 12 or 14 GPa to ambient pressure correlates well with a r u t i l e s t r u c t u r e ; t h e r e f o r e , t h e C N c h a n g e i s c o m p l e t e d b e l o w 1 2 G P a . D e c o m p r e s s i o n f a c i l i t a t e s a r e v e r s e t r a n s i t i o n , o c c u r i n g g r a d u a l l y b e l o w 6 G P a . U p o n c o m p l e t e r e l e a s e o f pressure, the Ge-O distance is 1.78 Å, s l i g h t l y l a r g e r t h a n t h a t i n t h e i n i t i a l q - G e O 2 a n d s i m i l a r t o t h a t o f g - G e O 2 . T h e C N c h a n g e o f g - G e O 2 b e g i n s above 6 GPa to finish below 10 GPa, a l o w e r p r e s s u r e t h a n n e c e s s a r y f o r q - GeO 2 by 2 GPa. The reversal change of CN occurs below 10 GPa for g-GeO 2 , wit h a sma lle r hys ter esi s tha n q-G eO 2 . At the complete release of pressure, the Ge -O di st an ce is 1. 79 Å th e sa me as observed for the untreated g-GeO 2 . As the g-GeO 2 originally includes a fraction o f t h e G e p o s s e s s i n g a s i x f o l d coordination, we expected a continuous 0 5 10 15 CN =4 Pressure ( GPa ) Ge-O in Rutile Ge-O distances (Å) CN =6 1.70 1.75 1.80 1.85 1.90 q-GeO 2 comp q-GeO 2 decomp g-GeO 2 comp g-GeO 2 decomp i n c r e a s e i n C N w i t h i n c r e a s i n g c o m p r e s s i o n . T h e Ge -O bo nd , ho we ve r, gr ad ua ll y sh or te ns be lo w 6 GPa and elongates within a narrow pressure range only. These results indicate that the compression of the GeO 4 tetrahedron is energetically less costly than CN change, even under circumstances where the GeO 6 octahedron nuclei preexists. T h e v a r i a t i o n o f σ 2 o f q - G e O a n d g - G e O 2 reveals that the initial value of g-GeO 2 is larger than that of q-GeO , reflecting the glassy state ( Fig. 3 ). Compression results in the increase of σ 2 for both q - a n d g - G e O 2 , c o r r e s p o n d i n g t o t h e c h a n g e s i n CN [5] . Decompression leaves the once-increased σ 2 o f q - G e O 2 a t s i m i l a r v a l u e s . T h e r e v e r s a l c h a n g e i n g - G e O 2 2 2 o c c u r s b e l o w 1 0 G P a w i t h h y s t e r e s i s . S e v e r a l a r g u m e n t s q u e s t i o n w h e t h e r t h e h i g h p r e s s u r e p h a s e i s a m o r p h o u s o r References [1] J.P. Itie et al. , Phys. Rev. Lett. 63 (1989) 398; D . J . D u r b e n a n d G . H . W o l f , P h y s . R e v . B 4 3 [1991) 2355. [2] J. Haines et al. , Phys. Rev. B 61 (2000) 8701. [ 3 ] S . S a k k a a n d K . K a m i y a , J . N o n - C r y s t a l l i n e Solids 49 (1982) 103. [4] O. Ohtaka, A. Yoshiasa, H. Fukui, K. Murai, M. Okube, Y. Katayama, W. Utsumi and Y. Nishihata, J. Synchrotron Rad. 8 (2001) 791. [5] A. Yoshiasa et al. , J. Synchrotron Rad. 6 (1999) 1051. cr ys ta ll in e. Pr es en t σ 2 of 0. 00 8 Å 2 fo r q- Ge O 2 at p r e s s u r e s g r e a t e r t h a n 1 0 G P a i s c o n s i d e r a b l y l a r g e r t h a n t h e v a l u e o f 0 . 0 0 4 Å 2 f o r r u t i l e - t y p e G e O 2 [ 5 ] ; i t i s s m a l l e r , h o w e v e r , t h a n t h a t o f g - G e O 2 i n t h e s a m e p r e s s u r e r a n g e . T h i s r e s u l t suggests that the local structure around Ge atoms w i t h i n t h e h i g h p r e s s u r e p h a s e i s m o r e r a n d o m than the rutile phase and less random than g-GeO 2 possessing sixfold coordination. As the CN change o c c u r s a t l o w t e m p e r a t u r e s d u r i n g w h i c h t h e the rma lly acti vate d dif fusi on pro ces s is res trai ned , t h e t r a n s i t i o n l i k e l y o c c u r s b y a m a r t e n s i t i c m e c h a n i s m . C o n s e q u e n t l y , w e p r o p o s e t h a t t h e h i g h p r e s s u r e p h a s e c o n s i s t s o f a f i n e c r y s t a l l i n e d o m a i n p o s s e s s i n g m a n y d e f e c t s . T h e r e l a t i v e l y l a r g e v a l u e o f σ 2 i n t h i s p h a s e i n d i c a t e s p o o r crystallization, resulting from defects introduced by a martensitic transition. Osamu Ohtaka Osaka University E-mail: ohtaka @ ess.sci.osaka-u.ac.jp
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