Bonding Nature in a Novel Superconductor, MgB2 | SPring-8/SACLA Research Frontiers

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I n t h e b e g i n n i n g o f t h e 2 1 s t c e n t u r y , a n o v e l superconductor, MgB 2 , was discovered by Akimitsu [1]. Because of its comparatively high Tc, 39 K, an anomalous number of experimental and theoretical s t u d i e s w e r e p e r f o r m e d i n o r d e r t o g a i n a n u n d e r s t a n d i n g o f t h e m e c h a n i s m o f s u p e r c o n d u c t i v i t y . Since 1954 [2], it has been known that the crystal st ru ct ur e of th is ma te ri al is he xa go na l ( Al B 2 ty pe , s p a c e g r o u p P 6 / m m m ) . T h e c h a r a c t e r i s t i c b o r o n h o n e y c o m b s h e e t s a r e s a n d w i c h e d b e t w e e n t h e Mg triangular sheets like an intercalated graphite as shown in Fig. 1 . The band structure calculations [3] pr ed ic t th e ex is te nc e of a ch ar ge do na ti on of tw o e l e c t r o n s f r o m t h e i o n i z e d M g t o t h e b o r o n c o n d u c t i o n b a n d w h i l e a s t r o n g B - B c o v a l e n t b o n d i n g i s r e t a i n e d . T h e s u p e r c o n d u c t i v i t y i n M g B 2 , w h i c h a p p e a r s e s s e n t i a l l y t o c o m e a b o u t d u e t o t h e metallic nature of the Boron 2-D sheets, has been i n t e r p r e t e d a s a p h o n o n - m e d i a t e d B C S - t y p e mechanism. Such a two dimensional structure is a com mon fea tur e in oxi de sup erc ond uct ors as wel l as in intercalated graphite. The doping on the Mg site or Boron 2-D sheets was carried out to reveal t h e e f f e c t o f t h e e l e c t r o n c o n c e n t r a t i o n o n t h e s u p e r c o n d u c t i n g t e m p e r a t u r e . S e v e r a l r e p o r t s o f the loss of superconductivity have been presented f o r M g 1 - x A l x B 2 , M g B 1 - x C x , M g 1 - x L i x B 2 a n d M g 1 - x M n x B 2 . Bonding Nature in a Novel Superconductor, MgB 2 T h e p r e s s u r e e v o l u t i o n o f t h e s u p e r c o n d u c t i n g t r a n s i t i o n t e m p e r a t u r e w a s a l s o r e p o r t e d a n d discussed for its relationship with the B-B and Mg-B b o n d i n g d i s t a n c e s . H o w e v e r , t h e s t r u c t u r a l info rmat ion used in the disc ussi ons was limi ted to t h e a t o m i c l e v e l , i . e . l a t t i c e c o n s t a n t s , b o n d i n g d i s t a n c e , e t c . A n e x p e r i m e n t a l c h a r g e d e n s i t y w o u l d g i v e a b e t t e r u n d e r s t a n d i n g o f h o w s u p e r c o n d u c t i v i t y links to the electronic and crystal structure of MgB 2 . I n 2 0 0 1 , w e r e p o r t e d o n t h e p r e c i s e c h a r g e d e n s i t i e s o f M g B 2 a t R . T . a n d 1 5 K u s i n g s y n c h r o t r o n r a d i a t i o n p o w d e r d a t a a n d p r e s e n t e d e x p e r i m e n t a l e v i d e n c e f o r s t r o n g B - B c o v a l e n t b o n d i n g , f u l l i o n i z a t i o n o f M g a t o m s a t b o t h t e m p e r a t u r e s a s w e l l a s c h a r g e c o n c e n t r a t i o n o n b o r o n 2 - D s h e e t a t 1 5 K , w h i c h m o s t p r o b a b l y relates to the superconducting mechanism [4]. T h e M g B 2 s a m p l e u s e d i n t h i s w o r k w a s prepared by Prof. J. Akimitsu. A pressed pellet of s t o i c h i o m e t r i c a m o u n t s o f M g a n d a m o r p h o u s B was heated for 10 hours at 700 C under an argon pressure of 196 MPa. The sample was found to be superconducting with T c = 39 K. The granularity of th e po wd er wa s re du ce d to a di am et er ev en le ss than 3 microns by the precipitation method in order to obtain homogeneous intensity distribution in the Debye-Scherrer powder ring. The obtained powder sa mp le wa s se al ed in a si li ca gl as s ca pi ll ar y (0 .3 m m i n t . d i a m . ) . T h e s y n c h r o t r o n r a d i a t i o n X - r a y p o w d e r e x p e r i m e n t w i t h i m a g i n g p l a t e s ( I P ) a s d e t e c t o r s w a s c a r r i e d o u t b y t h e L a r g e D e b y e - Scherrer Camera at beamline BL02B2 [5]. The He g a s c i r c u l a t i o n t y p e c r y o s t a t w a s u s e d f o r t h e me as ur em en t at lo w te mp er at ur e ( Fi g. 2 ). Th e X- r a y p o w d e r p a t t e r n s w e r e m e a s u r e d a t r o o m t e m p e r a t u r e ( R . T . ) a n d 1 5 K ( < < T c ) . B o t h d a t a w e r e o b t a i n e d u n d e r t h e s a m e e x p e r i m e n t a l c o n d i t i o n s e x c e p t f o r t h e t e m p e r a t u r e . T h e e x p o s u r e t i m e w a s 1 h o u r . T h e w a v e l e n g t h o f i n c i d e n t X - r a y s w a s 0 . 6 Å . T h e X - r a y p o w d e r p a t t e r n o f M g B 2 w a s o b t a i n e d w i t h a 0 . 0 2 s t e p from 9.0 to 65.0 in 2 θ , which corresponds to 0.57 Å resolution in d- spacing. Fig. 1. Crystal structure of MgB 2 . 30 L a t i c e P a r a m e t e r ( Å ) B-B distance (Å ) Mg-B distance (Å ) a = 3.08831(3) a = 3.08365(2) c = 3.52415(8) c = 3.51504(4) 1.78304(8) 1.78035(1) 2.50170(1) 2.50682(1) R.T. 15 K . T h e l a t t i c e p a r a m e t e r s a n d B - B , M g - B atomic distances determined by the Rietveld analysis for MgB 2 at R.T. and 15 K. Fig. 2. The photograph of the displex installed in the Large Debye-Scherrer Camera. 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 s a t b o t h temperatures were visualized by the MEM/ Rietveld 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 a n d the Rietveld refinement [6]. This method has been succ essf ull y appl ied to the char ge dens ity stud ies of fu ll er en e co mp ou nd s, in te rm et al li c co mp ou nd s, α - b o r o n , m a n g a n i t e , e t c . F o r i n s t a n c e , t h e MEM / Rie tve ld met hod usi ng syn chr otr on rad iat ion powder data has revealed Mn 3d x 2 — y 2 orbital order as a Mn-O bonding electron distribution associated w i t h M n ( 3 d ) - O ( 2 p σ ) h y b r i d i z a t i o n a t a n t i f e r r o m a g n e t i c state in manganite, NdSr 2 Mn 2 O 7 [7]. In the present p o w d e r d a t a , s e v e r a l w e a k i m p u r i t y p e a k s w e r e f o u n d a n d i d e n t i f i e d a s M g O . T h e i m p u r i t y M g O phase was also taken into account in the Rietveld p r e - a n a l y s i s . T h e s p a c e g r o u p w a s a s s i g n e d t o P 6 / m m m f o r b o t h d a t a a t R . T . a n d 1 5 K . T h i s imp lie s tha t the re is no str uct ura l pha se tra nsi tio n Fig. 3. Fitting results of Rietveld analysis of MgB 2 at (a) R.T. and (b) 15 K. 3.1% and 3.4% for R.T. and 15 K, respectively. In t h e a n a l y s i s , t h e s t r u c t u r e f a c t o r s o f t h e 5 5 r e f l e c t i o n s w e r e d e r i v e d f r o m t h e o b s e r v e d i n t e g r a t e d i n t e n s i t i e s . T h e y w e r e t h e n u s e d f o r fur the r MEM ana lys is. Fol low ing the Rie tve ld pre - analysis, the MEM analysis was carried out by the computer program, ENIGMA [8], using 64 × 64 × 72 pixels. The reliable factors of the final MEM charge d e n s i t y w e r e 1 . 7 % a n d 1 . 5 % f o r R . T . a n d 1 5 K , respectively. from R.T. to 15 K. The results of the Rietveld r e f i n e m e n t a r e s h o w n f o r R . T . a n d 1 5 K i n Fig. 3(a) and 3(b) , respectively. The refined lattice parameters are listed in Table I . The w e i g h t e d p r o f i l e r e l i a b i l i t y f a c t o r s o f t h e Rietveld refinement as a pre-analysis for the MEM, R WP , were 4.7% and 2.6% for R.T. and 15 K, respectively. And the re li ab il it y fa ct or s based on the integra ted intensi ties, R I , we re (a) (b) 2 θ θ (degree) 10 20 30 40 50 60 1 0 0 0 0 0 0 2 0 0 0 0 0 3 0 0 0 0 0 Intensity (arbitrary units) Intensity (arbitrary units) 1 0 0 0 0 0 0 2 0 0 0 0 0 3 0 0 0 0 0 2 θ θ (degree) 10 20 30 40 50 60 R WP = 2.6% R I = 3.4% R WP = 4.7% R I = 3.1% (a) (b) 31 A three-dimensional representation of the final MEM charge density at R.T. is shown in Fig. 4 as an equ i-c har ge den sity sur face . The equ i-d ens ity l e v e l i s 0 . 7 5 e / Å 3 . T h e o b t a i n e d M E M c h a r g e de ns it y cl ea rl y ex hi bi ts a st ro ng co va le nt bo nd in g n e t w o r k o f b o r o n 2 - D s h e e t f o r m i n g t h e s i x - membered rings, which are colored in blue. On the ot he r ha nd , th er e is no lo ca li ze d el ec tr on de ns it y between Mg and boron atoms. In the interatomic r e g i o n , e l e c t r o n s a r e d i s t r i b u t e d r a t h e r e v e n l y s i m i l a r t o m e t a l b o n d i n g . T h e s e c h a r a c t e r i s t i c d e n s i t y f e a t u r e s a r e p r e s e r v e d i n t h e c h a r g e d e n s i t y o b t a i n e d a t R . T . a n d c o n s i s t e n t w i t h t h e calculated band structures indicating the two band model [3]. Based on this model, several theoretical m e c h a n i s m s o f s u p e r c o n d u c t i v i t y h a v e b e e n proposed [9]. In Fig. 5 , the MEM charge densities of the (110) sections containing Mg and boron atoms are shown f o r R . T . a n d 1 5 K w i t h a s t r u c t u r e m o d e l . T h e contour lines are drawn only for the lower density reg ion . It is con fir med tha t the re is no sig nif ica nt o ve r l a p p i n g o f th e ch a r g e d e n si ty a r o u n d th e M g atomic sites. This is a high contrast to that of the boron -boro n netwo rk. The MEM charg e densi ties c l e a r l y r e v e a l t h e b o r o n - b o r o n c o v a l e n t b o n d i n g Fig. 4. The equi-contour (0.75 e/Å 3 ) surface of MEM charge density of MgB 2 at R.T. features. Although the change in the boron-boron i n t e r a t o m i c d i s t a n c e i s e x t r e m e l y s m a l l b e t w e e n R . T . a n d 1 5 K a s s h o w n i n T a b l e I , t h e c h a r g e d e n s i t y v a l u e s a t t h e b o n d m i d p o i n t s s h o w t h e distinct different values, which are 0.9 and 1.0 eÅ -3 at R.T. and 15 K, respectively. These values are in the range between those of Si ( 0. 7e Å -3 ) [6 ] an d Di am on d (1 .4 eÅ -3 ) [6 ] an d ve ry close to the value of hexagonal-BN (1.0 eÅ -3 ) [10]. T h e v a l e n c e o f t h e a t o m w a s e x a m i n e d b y a c c u m u l a t i n g t h e n u m b e r o f e l e c t r o n s a r o u n d a c e r t a i n a t o m i n t h e M E M d e n s i t y . S o f a r , t h e v a l e n c e o f m e t a l a t o m s e n c a p s u l a t e d i n m e t a l l o f u l l e r e n e h a s b e e n d e t e r m i n e d experimentally from the MEM charge densities [6]. The number of electrons around an Mg atom was e s t i m a t e d a s a b o u t 1 0 . 0 ( 1 ) a n d 1 0 . 0 ( 1 ) , r e s p e c t i v e l y . T h e s e v a l u e s a r e v e r y c l o s e t o t h e number of electrons of Mg 2 + ion. This means that th e Mg at om s ar e fu ll y io ni ze d in Mg B 2 cr ys ta l at both R.T. and 15 K. On the other hand, the number o f e l e c t r o n s b e l o n g i n g t o t h e b o r o n 2 - D s h e e t s s h o w s i g n i f i c a n t d i f f e r e n c e , t h e y a r e 9 . 9 ( 1 ) e e e a n d 10.9(1) e at R.T. and 15 K, respectively. This can be int erp ret ed as the val enc e of the who le b o r o n 2 - D s h e e t c h a n g i n g f r o m n e u t r a l t o m o n o v a l e n t , 32 (a) (b) Eiji Nishibori, Masaki Takata and Makoto Sakata Nagoya University E-mail: a41024a @ nucc.cc.nagoya-u.ac.jp i . e . , ( B - B ) — a t 1 5 K , w h i c h g i v e s e v i d e n c e o f t h e increase of charge at the B-B bond midpoint at 15 K . T h o u g h t h e f u l l c h a r g e t r a n s f e r f r o m M g t o boron 2-D sheets was expected to occur and form a n i s o e l e c t r o n i c s h e e t w i t h g r a p h i t e , n o s i m p l e di re ct fu ll ch ar ge tr an sf er fr om Mg 2+ to bo ro n 2- D shee t was obse rved . The pres ent resu lts supp ort t h e f o l l o w i n g s c e n a r i o , t h a t i s , t h a t t h e v a l e n c e electrons are delocalized in the inter-atomic region at R.T., and half of them l o c a l i z e d o n t h e b o r o n 2 - D s h e e t s a t l o w t e m p e r a t u r e . This scenario implies the p r e s e n c e o f e l e c t r o n t r a n s f e r f r o m t h e π b o n d s c o n s i s t i n g o f p z o r b i t a l s t o i n - p l a i n σ b o n d s consisting of p xy orbitals in the two band model of M g B 2 a t 1 5 K . C o n s e q u e n t l y , a s u b t l e b u t important charge concentration on boron 2-D sheet at 15 K was found, which most probably relates to the superconductivity of this compound. F i g . 5 . T h e ( 1 1 0 ) s e c t i o n s o f t h e M E M C h a r g e Density of MgB 2 at (a) R.T. and (b) 15 K with the sch ema tic rep res ent ati on of the cry sta l str uct ure . T h e c o n t o u r l i n e s a r e d r a w n f r o m 0 . 0 t o 3 . 9 a t 0.15(e/Å 3 ) intervals for four unit cells. References [1] J. Nagamatsu et al. , Nature 410 (2001) 63. [2] M.E. Jones and R.E. Marsh, J. Am. Chem. Soc. 76 (1954) 1434. [ 3 ] J . K o r t u s e t a l . , P h y s . R e v . L e t t . 8 6 ( 2 0 0 1 ) 4656; preprint cond-mat / 0101446. [4] E. Nishibori et al. , J. Phys. Soc. Jpn. 70 (2001) 2252. [5] E.Nishibori et al. , Nucl. Instrum. Meth. in Phys. Res. A 467-468 (2001) 1045. [6] M. Takata, B. Umeda, E. Nishibori, M. Sakata, M. Ohno and H. Shinohara, Nature 377 (1995) 46; M . T a k a t a , E . N i s h i b o r i a n d M . S a k a t a , Z . Kristallogr. 216 (2001) 71. [7] M. Takata et al. , J. Phys. Soc. Jpn. 68 (1999) 2190. [ 8 ] H . T a n a k a , e t a l . , J . A p p l . C r y s t . ( 2 0 0 2 ) - i n press. [9] M. Imada, J. Phys. Soc. Jpn. 70 (2001) 1218; K. Yamaji, J. Phys. Soc. Jpn. 70 (2001) 1476. [10] S. Yamamura et al. , J. Phys. Chem. Solids 58 (1997) 177. 33