Intensity (counts) Intensity (counts) 2 θ (degree) 18 19 20 6000 5000 4000 3000 {111} {210} MgO 2 θ θ (degree) 10 20 30 40 50 60 70 0 50 100 150 200 × 10 3 C h e m i c a l B o n d i n g o f H y d r o g e n i n C h e m i c a l B o n d i n g o f H y d r o g e n i n M g H 2 Hy dr og en is co ns id er ed to be on e of le ad in g candidates for clean energy sources in the future. F o r s a f e a n d e f f i c i e n t h y d r o g e n s t o r a g e , de ve lo pm en ts of ne w hy dr og en st or ag e ma te ri al s are currently being researched [1]. The hydrogen s t o r a g e c a p a c i t y p e r u n i t w e i g h t o f t y p i c a l m e t a l a l l o y s i s v e r y l o w ( a b o u t 2 . 0 m a s s % ) a n d n o t sufficient for use in a fuel cell vehicle. Therefore, alloys containing light elements are focused on as h i g h - p e r f o r m a n c e s t o r a g e m a t e r i a l s . M a g n e s i u m in particular has a high storage capacity (7.6 mass %). For this reason, magnesium and its alloys are c o n s i d e r e d t o b e s o m e o f t h e m o s t i m p o r t a n t c a n d i d a t e s f o r r e v e r s i b l e h y d r o g e n s t o r a g e ma te ri al s . Un fo rt un at el y, ma gn es iu m hy dr id es ar e th er mo dy na mi ca ll y st ab le , an d th e de hy dr og en at io n of magnesium hydrides requires high temperatures ( > 5 5 0 K ) . U n d e r s t a n d i n g t h e b o n d i n g n a t u r e o f m a g n e s i u m a n d h y d r o g e n i s e s s e n t i a l i n o r d e r t o i m p r o v e i t s f u n d a m e n t a l d e h y d r o g e n a t i o n performance. T o f u r t h e r u n d e r s t a n d t h e n a t u r e o f b o n d i n g , cha rge den sit y dis tri but ion is typ ica lly inv est iga ted b y X - r a y d i f f r a c t i o n a n a l y s i s . H o w e v e r , t h e di ff ra ct io n in te ns it y fr om hy dr og en at om s in me ta l hydrides is very weak , although we have overcome th at di ff ic ul ty wi th th e hi gh ly br il li an t X- ra y so ur ce of SPring-8. In our result, the diffraction intensities f r o m h y d r o g e n c o u l d b e m e a s u r e d p r e c i s e l y a n d t h e p o s i t i o n s o f h y d r o g e n i n m e t a l h y d r i d e s w e r e determined by the analysis based only on the X-ray d i f f r a c t i o n d a t a . F u r t h e r m o r e , t h e M E M / R i e t v e l d m e t h o d , w h i c h i s a c o m b i n a t i o n o f t h e M E M ( m a x i m u m e n t r o p y m e t h o d ) a n d t h e R i e t v e l d r e f i n e m e n t [ 2 ] , was applied to metal hydrides. By employing this m e t h o d a n d p r e c i s e X - r a y d i f f r a c t i o n d a t a , t h e c h a r g e d e n s i t y d i s t r i b u t i o n m e a s u r e m e n t , w h i c h has not so far succeeded , became possible fo r th e f i r s t t i m e , a n d t h e b o n d i n g n a t u r e o f h y d r o g e n i n MgH 2 was revealed experimentally [3]. The MgH 2 sample was prepared by hydrogen a c t i v a t i o n t r e a t m e n t o f p u r e m a g n e s i u m p o w d e r . Fig. 1. Rietveld analysis pattern of the MgH 2 sample. Red + marks are measured intensity, the b l u e l i n e i s c a l c u l a t e d i n t e n s i t y , b l a c k | m a r k s a r e p e a k p o s i t i o n s , a n d t h e p u r p l e l i n e i s t h e de vi at io n of me as ur em en t an d ca lc ul at ed in te ns it y. Mg H 2 , Mg an d Mg O ar e in cl ud ed in th is sample. The enlarged pattern of the region from 18.0 to 20.5 is shown in the inset. 78 H Mg (001) (110) a b c H Mg Fig. 2. The crystal structure of MgH 2 (rutile type). The obtained fine powder was inserted into a glass capillary (diameter: 0.1 mm ) for measurement and t h e 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 w a s c a r r i e d o u t using a large Debye-Scherrer camera with imaging p l a t e a s d e t e c t o r a t b e a m l i n e B L 0 2 B 2 [ 4 ] . T h e wavelength of the incident X-ray was 0.696 Å. The X - r a y d i f f r a c t i o n i n t e n s i t i e s w e r e o b t a i n e d w i t h 0.02 s t e p s f r o m 5 . 0 t o 7 3 . 0 i n 2 θ . T h e e x p e r i m e n t a l da ta we re an al yz ed by th e ME M/ Ri et ve ld me th od u s i n g t h e c o m p u t e r p r o g r a m s E N I G M A [ 5 ] a n d RIETAN [6]. T h e p o w d e r s a m p l e w a s c o m p o s e d o f t h r e e phases, MgH 2 , Mg and MgO. By Rietveld analysis, the mass fractions for each phase were determined as 69, 27 and 4 mass%, respectively. The lattice p a r a m e t e r o f M g H 2 i s a = 4 . 5 1 8 0 ( 6 ) Å , c = 3 . 0 2 1 1 ( 4 ) Å , w h i c h i s a l i t t l e l a r g e r t h a n t h a t o f MgD 2 . The weighted profile reliability factor, R WP , a n d t h e r e l i a b i l i t y f a c t o r b a s e d o n i n t e g r a t e d inte nsit ies, R I , of the Riet veld anal ysis were 2.9% a n d 1 . 7 % , r e s p e c t i v e l y . T h e f i t t i n g r e s u l t o f t h e Rietveld analysis is shown in Fig. 1 . The inserted f i g u r e c l e a r l y s h o w s t h e { 1 1 1 } r e f l e c t i o n a t 1 8 . 2 a n d t h e { 2 1 0 } r e f l e c t i o n a t 1 9 . 8 , w h i c h a r e attributed to the sub-lattice of only H atoms. I n M E M a n a l y s i s , t h e r e l i a b i l i t y f a c t o r R w a s 1 . 0 % , w h i c h i s s m a l l e n o u g h t o d e t e r m i n e t h e charge densities of hydrogen and its bond nature. F i g u r e 2 i l l u s t r a t e s t h e c r y s t a l s t r u c t u r e o f M g H 2 ( r u t i l e t y p e ) . T h e c h a r g e d e n s i t y d i s t r i b u t i o n o f ( 0 0 1 ) a n d ( 1 1 0 ) p l a n e i n t h e c r y s t a l o f M g H 2 i s s h o w n i n F i g . 3 ( a ) a n d F i g . 3 ( b ) . T h e c h a r g e density distribution around Mg is spherical, whereas t h e l o w e r c h a r g e d e n s i t y d i s t r i b u t i o n a r o u n d H i s no n- sp he ri ca l an d sp re ad s sl ig ht ly in th e di re cti on of th e ne ar es t ne ig hb or Mg an d H at om s. At th e bond midpoint, the charge density is 0.26, 0.21 and 0.25 e/Å 3 for the apical Mg–H, the equatorial Mg–H and the H–H bonds, respectively. These values are much lower than those of typical covalent bond s in d i a m o n d ( 1 . 4 e / Å 3 ) a n d S i ( 0 . 7 e / Å 3 ) [ 2 ] . T h e results suggest that there are weak but significant c o v a l e n t b o n d s b e t w e e n M g a n d H a s w e l l a s b e t w e e n H a n d H . T h e c h a r g e d e n s i t y i n t h e i n t e r s t i t i a l r e g i o n i s e x t r e m e l y l o w , 0 . 0 2 e / Å 3 , w h i c h c o u l d d e n y t h e e x i s t e n c e o f t h e m e t a l l i c b o n d i n g n a t u r e . T h e n u m b e r o f e l e c t r o n s w i t h i n t h e spherical region was estimated as 10.09 e and 1.26 e fo r Mg (r ad iu s: 0. 95 Å) an d H (ra di us : 1. 00 Å) . These values indicate that ionic charge of Mg and 79 Fig. 3. MEM charge density maps of (001) and (110) plane in MgH 2 . The contour lines are drawn from 0.0 to 1.5 at 0.15 e/Å 3 intervals. References [1] L. Schlapbach and A. Züt tel, Nature 414 (2001) 353. [2] M. Takata et al. , Z. Krystallogr. 216 (2001) 71. [ 3 ] T . N o r i t a k e , M . A o k i , S . T o w a t a , Y . S e n o , Y . Hir ose, E. Nish ibor i, M. Taka ta, M. Saka ta, Appl . Phys. Lett. 81 (2002) 2008. [4] E. Nishibori et al. , Nucl. Instrum. Meth. in Phys. Res. A 467-468 (2001) 1045. [5] F. Izumi, The Rietveld Method, Ed. R. A. Young (Oxford Univ. Press, 1993) chap. 13 . [6] H. Tanaka et al. , J. Appl. Crystallogr. 35 (2002) 282. [ 7 ] K . M i w a a n d A . F u k u m o t o , P h y s . R e v . B 6 5 (2002) 155114. Tatsuo Noritake Toyota Central R & D Labs., Inc. E-mail: e0553 @ mosk.tytlabs.co.jp H are represented as Mg 1.91 + and H 0.26– . Hydrogen is far more weakly ionized than magnesium. Experimentally , the bonding nature of MgH 2 has b e e n r e v e a l e d b y t h e c h a r g e d e n s i t y d i s t r i b u t i o n me as ur em en t. As a re su lt , we co ns id er th at it is nece ssar y to make its ioni c bond ing even weak er t o i m p r o v e t h e d e h y d r o g e n a t i o n p e r f o r m a n c e o f MgH 2 . In the future, it will be necessary to find an effective method to weaken the bonding strength by the selection of a suitable crystal structure, adding e l e m e n t s a n d s o o n . T h e e l e c t r o n i c s t a t e s a n d f o r m a t i o n e n e r g y o f m e t a l h y d r i d e s h a v e b e e n t h e o r e t i c a l l y c a l c u l a t e d [ 7 ] . T h e n e x t s t e p i s t o u n d e r t a k e e x p e r i m e n t a l a n d t h e o r e t i c a l i n v e s t i g a t i o n s i n t o t h e b o n d i n g n a t u r e o f h y d r o g e n t o d e v e l o p efficient hydrogen storage materials. c a [110] b 0.0 0.75 1.5 e/ Å 3 80
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