Temperature (K) Pressure ( GPa ) 300 100 200 0 100 30 50 150 200 Phase III Phase II Phase I X-ray Powder Diffraction from Solid Deuterium Recent optical studies have demonstrated that s o l i d h y d r o g e n e x h i b i t s t h r e e c r y s t a l l i n e p h a s e s mainly based on the difference in the orientation of m o l e c u l e s ( F i g . 1 ) [ 1 ] . I n t h e d i a g r a m , o n l y t h e cr ys ta ll in e st ru ct ur e of p ha se I h as b ee n de te rm in ed . S tr u c tu r a l s tu d i e s o f s o l i d h y d r o g e n b y X - r a y d i f f r a c t i o n t e c h n i q u e a r e s e v e r e l y r e s t r i c t e d because of the low scattering efficiency of hydrogen and the small size of the sample at high pressure. B e c a u s e t h e i n t e n s i t y f r o m a s i n g l e c r y s t a l r e f l e c t i o n i s h i g h e r t h a n t h a t f o r a p o l y c r y s t a l l i n e sample , the X-ray diffra ction exper iment with solid hydrogen was first carried out by the use of single- c r y s t a l t e c h n i q u e a t 5 . 4 G P a , a n d t h e c r y s t a l structure of phase I was determined to be hcp [2]. Me as ur em en ts at hi gh er pr es su re s we re im pe de d by the drastic reduction in diffracted intensity due to a significant reduction in the volume of the sample c h a m b e r a n d / o r t h e f r a g m e n t a t i o n o f a c r y s t a l un de r pr es su re . Lo ub er yr e et al . ha ve ov er co me the difficulty with the excellent technique of growing a single crystal in helium and the application of high b r i l l i a n c e t h i r d g e n e r a t i o n s y n c h r o t r o n X - r a y s o u r c e s . B y t h e s e m e a n s , t h e p r e s s u r e - v o l u m e relation of solid hydrogen and deuterium have been determined to 120 GPa at room temperature [3]. In spite of the low intensity from a polycrystalline s a m p l e , t h e p o w d e r d i f f r a c t i o n t e c h n i q u e i s s t i l l important because it is simple and convenient, and m o r e o v e r , s i n g l e c r y s t a l s f r e q u e n t l y f r a g m e n t a t structural transition accompanying a discontinuous change in volume. In this work, our interest is focussed on whether o r n o t t h e p o w d e r t e c h n i q u e c a n b e u s e d f o r s t r u c t u r a l s t u d i e s o f s o l i d h y d r o g e n a t h i g h p r e s s u r e s and to determine the crystalline structure of phase II . Th e tr an si ti on bo un da ry be tw ee n ph as e I an d p h a s e I I e x h i b i t s a s t r o n g i s o t o p e e f f e c t a n d t h i s shifts to a higher temperature and a lower pressure Fig. 1. Schematic phase diagram of solid hydrogen. T h e d a s h e d l i n e r e p r e s e n t s t h e b o u n d a r y b e t w e e n phase I and phase II for solid deuterium. for deuterium (dotted curve in Fig. 1 ) [1,4,5]. The p o w d e r X - r a y d i f f r a c t i o n e x p e r i m e n t s o f s o l i d deuterium were carried out in this way. A d i a m o n d a n v i l c e l l ( D A C ) , w i t h a t u n g s t e n c a r b i d e h e m i s p h e r i c s e a t i n g a n d a c o n e - s h a p e d ap er tu re to de te ct di ff ra ct ed X- ra ys , wa s us ed fo r hig h pre ssu re gen era tio n. The top sur fac e of the anvil was 0.3 mm. An Re gasket was pre-indented to 50 μ m and a hole of 110 μ m diameter was made a s a s a m p l e c h a m b e r . T h e h i g h - p r e s s u r e e x p e r i m e n t s w e r e c a r r i e d o u t a t b e a m l i n e B L 1 0 X U . T h e w a v e l e n g t h w a s t u n e d w i t h a n S i ( 1 1 1 ) d o u b l e - c r y s t a l m o n o c h r o m a t o r t o 0 . 6 1 9 6 Å . A n X - r a y refractive lens made of molded PMMA ( polymethyl methacrylate, density 1.19 g/cc) was inserted in the X- ra y pa th to en ha nc e th e de ns it y of th e in ci de nt beam. The sample was exposed to an X-ray beam t h r o u g h a p i n h o l e c o l l i m a t o r o f 2 6 μ m d i a m e t e r . The cell was oscillated within 5 deg. The typical e x p o s u r e t i m e w a s 3 0 m i n . T h e p o w d e r p a t t e r n s were obtained by an angle-dispersive method with an image plate detector. F i g u r e 2 s h o w s t w o - d i m e n s i o n a l d i f f r a c t i o n images from solid deuterium at 62.3 GPa and 300 K . I n t h e f i g u r e , t w o d i f f e r e n t a r e a s a r o u n d t h e D e b y e - S c h e r r e r r i n g s a r e s e l e c t e d b e c a u s e t h e 41 Fig. 3. Integrated one-dimensional diffraction pattern obtained at 62.3 GPa and 300 K. The inset shows an expanded scale of intensity between 2 θ = 19 and 25 deg. Intensity ( arb. ) 20 22 24 D 2 ( 1 0 0 ) D 2 ( 0 0 2 ) D 2 ( 1 0 1 ) 2 θ θ ( deg. ) 10 20 30 λ λ = 0.6196 Å R e D x ( 1 1 0 ) R e D x ( 1 0 2 ) R e D x ( 1 0 0 ) R e D x ( 1 0 1 ) Haruki Kawamura and Yuichi Akahama Himeji Institute of Technology E-mail: kawamura @ sci.himeji-tech.ac.jp References [1] H. K. Mao and R. J. Hemley, Rev. Mod. Phys. 66 (1994) 671. [ 2 ] R . M . H a z e n e t a l . , P h y s . R e v . B 3 6 ( 1 9 8 7 ) 3844. [ 3 ] P . L o u b e y r e e t a l . , H . K . M a o , L . W . F i n g e r , Nature 383 (1996) 702. [4] A.F. Goncharov et al. , Phys. Rev. B 54 (1996) R15590. [5 ] I. I. Ma zi n e t al . , Ph ys . R ev . Le tt . 78 ( 19 97 ) 10 66 . dif fra cti on aro und the rin g was not uni for m. Thr ee d i f f r a c t i o n r i n g s f r o m s o l i d d e u t e r i u m c a n b e observed. These are assigned to the 100, 002 and 1 0 1 d i f f r a c t i o n r i n g s f r o m a n h c p l a t t i c e . T h e r e were strong arcs from the rhenium gasket. Figure 3 d e p i c t s t h e o n e - d i m e n s i o n a l d i f f r a c t i o n p a t t e r n i n t e g r a t e d a r o u n d t h e w h o l e r i n g . T h e s i g n a l - t o - n o i s e r a t i o o f t h e 1 0 1 d i f f r a c t i o n l i n e o f s o l i d deuterium was about 11. The lattice constants at 62.3 GPa and 300 K were a = 2.015 and c = 3.237 Å. The derived cell volume and the c / a ratio were consistent with single-crystal data [3]. The pressure cell was cooled to 83 K. During the course of cooling, pressure increased greatly to 94 GPa due to the thermal shrinkage of the body of t h e p r e s s u r e c e l l . T h e p r e s s u r e w a s e s t i m a t e d f r o m t h e p r e s s u r e - v o l u m e r e l a t i o n o f t h e r h e n i u m g a s k e t b e c a u s e t h e r u b y s i g n a l w a s l o s t i n m e a s u r e m e n t . T h e p r e s s u r e w a s s l i g h t l y overestimated because of the large compressibility of deuterium, the diffraction profile, however, should be ob ta in ed fr om ph as e II . Th re e di ff ra ct io n ar cs f r o m t h e s a m p l e c a n b e o b s e r v e d ( F i g . 4 ) , w h i c h a r e a l s o a s s i g n e d t o t h e 1 0 0 , 0 0 2 a n d 1 0 1 d i f f r a c t i o n l i n e s o f t h e h c p l a t t i c e . T h e l a t t i c e c o n s t a n t s a r e a = 1 . 9 6 4 a n d c = 3 . 1 4 5 Å ( c / a = 1.601). The center of each molecule is still on the hcp lattice point in phase II. Fig. 2. Two- dime nsio nal diff ract ion imag es from soli d deuterium at 62.3 GPa and 300 K. Two different areas ar ou nd th e De by e- Sc he rr er ri ng s ar e se le ct ed be ca us e the diffraction intensity was not uniform around the ring. Fig. 4. Two-dimensional diffraction images from solid deuterium at 94 GPa and 83 K. 42
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