Chapter 1 High-Temperature Superconductivity i n Oxides Arthur W. Sleight
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Central Research and Development Department, Ε. I. du Pont de Nemours and Company, Experimental Station, Building 356, Wilmington, DE 19898
The common structural feature of oxide superconductors is the presence of (M-O) chains which are linear or nearly so. In addition, mixed valency and high covalency are key factors for high temperature superconductivity in both the ΒaΡb1_ χΒiχO3 system and the new oxide superconductors
based on copper. The disproportionation reactions of Bi and Cu are especially c r u c i a l . Highly electropositive cations, such as B a2+, are required to sufficiently s t a b i l i z e oxygen 2p levels so that the Cu and Bi states can in turn be s t a b i l i z e d . High-temperature superconductors are normally expected to be metastable materials; the evidence for metastability in these oxide superconductors is presented. IV
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The discovery in 1975 of high-temperature superconductivity in the Β 3 Ρ ^ « Β ΐ 0 system (1) was widely hailed as a novel type of superconductivity, and new mechanisms have been proposed (2). The recently discovered oxide superconductors based on copper (3-6) are considered by many to be new examples of this type of superconductivity (4,5). The higher c r i t i c a l temperatures (T ) in the copper systems are presumably due to the fact that states near the Fermi level (E ) are primarily 3d in the copper system whereas they are primarily 6s in the case of the B a P b _ B i 0 system. Since d bands are generally narrower than s bands, a higher density of states at E is expected in the copper systems. A higher density of states at E has been observed experimentally and, according to BCS theory (7), this could account for higher T ' s in the copper systems. χ
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0097-6156/87/035l-0002$06.00/0 © 1987 American Chemical Society
In Chemistry of High-Temperature Superconductors; Nelson, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
1.
High-Temperature Superconductivity in Oxides
SLEIGHT
I t s h o u l d be e m p h a s i z e d , however, t h a t a l l t h e o x i d e s y s t e m s m e n t i o n e d have a much l o w e r d e n s i t y o f s t a t e s a t E than expected f o r such high T ' s . The T o f 13 Κ found i n the Β 3 Ρ ^ « Β ί 0 s y s t e m s t i l l s t a n d s as t h e r e c o r d f o r any m a t e r i a l n o t c o n t a i n i n g a t r a n s i t i o n metal. T h e r e a r e many m a t e r i a l s w i t h h i g h e r d e n s i t y o f s t a t e s at E b u t w i t h l o w e r T ' s t h a n i n t h e new copper oxide superconductors. In t h i s p a p e r I p o i n t o u t t h e s i m i l a r i t i e s and d i f f e r e n c e s between t h e Β 3 Ρ ^ _ Β ί 0 and t h e c o p p e r systems. T h i s w i l l be done f r o m a c h e m i s t r y v i e w p o i n t c o n s i d e r i n g s u c h i s s u e s as c o v a l e n c y , m i x e d v a l e n c y , s t r u c t u r e , and m e t a s t a b i l i t y . In an a t t e m p t t o be c o n s i s t e n t w i t h a c c e p t e d c h e m i s t r y n o m e n c l a t u r e , Roman n u m e r a l s u p e r s c r i p t s t o t h e r i g h t o f e l e m e n t s y m b o l s ( e . g . B i ) w i l l f r e q u e n t l y be u s e d t o i n d i c a t e o x i d a t i o n number and t o e m p h a s i z e t h a t t h i s number d o e s n o t s u g g e s t any p a r t i c u l a r d e g r e e o f ionization. T h i s p a p e r c o n t a i n s some d i s c u s s i o n o f t h e meaning of t h e s e s u p e r s c r i p t s . f
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Structure The o x i d e s s o f a r m e n t i o n e d i n t h i s p a p e r c a n a l l be c o n s i d e r e d t o have p e r o v s k i t e - r e l a t e d s t r u c t u r e s . In f a c t , the s t r u c t u r e f o r superconductors i n the Β3Ρ^_ Βΐ 0 system i s very c l o s e to the i d e a l c u b i c perovskite structure. The s t o i c h i o m e t r y i s e s s e n t i a l l y AM0 , and t h e r e i s j u s t a s m a l l t e t r a g o n a l d i s t o r t i o n f r o m c u b i c symmetry. The s t r u c t u r e o f t h e new o x i d e s u p e r c o n d u c t o r s b a s e d on c o p p e r ( i . e . L a ^ ^ A x C u 0 and R B a C u 0 . ) d e v i a t e much more f r o m t h e i d e a l p e r o v s k i t e structure. The L a C u 0 o r K N i F s t r u c t u r e may be v i e w e d as an i n t e r g r o w t h between t h e p e r o v s k i t e and s o d i u m c h l o r i d e s t r u c t u r e s , i . e . AO-AMO^ = A M 0 . However, t h i s r e s u l t s i n a l a y e r e d s t r u c t u r e w i t h M0 s h e e t s and no M-O-M bonding p e r p e n d i c u l a r to the s h e e t s . For the 1-2-3 compound, e.g. Y B a C u 0 . , t h e r e i s e x t e n s i v e o x y g e n deficiency, i.e. A M 0 i n s t e a d o f AM0 . The o x y g e n vacancies c r e a t e d are mostly ordered. This causes a s e v e r e d e p a r t u r e f r o m t h e p e r o v s k i t e s t r u c t u r e , and v i e w i n g R B a C u 0 as p e r o v s k i t e - r e l a t e d i s o f d u b i o u s value. N o n e t h e l e s s , a l l of the c a t i o n s i n the 1-2-3 s t r u c t u r e have t h e p e r o v s k i t e a r r a n g e m e n t ; R and Ba a r e o r d e r e d on A c a t i o n s i t e s , and Cu o c c u p i e s t h e M s i t e s . The o x y g e n v a c a n c i e s d r o p t h e c a t i o n c o o r d i n a t i o n numbers: R, 12->8, Ba; 12-+10; C u ( l ) , 6->5 and C u ( 2 ) , 6-*4. In t h e i d e a l p e r o v s k i t e s t r u c t u r e , t h e r e a r e i n f i n i t e l i n e a r -M-O-M-0- c h a i n s a l o n g a l l a x e s o f t h e unit c e l l . These chains i n t e r s e c t at the M c a t i o n s t o g i v e o c t a h e d r a l c o o r d i n a t i o n a r o u n d M. I t s h o u l d be noted t h a t the s t r u c t u r e of p r e v i o u s T record holders, e.g. N b G e , has s t r o n g l y bonded Nb c h a i n s a l o n g e a c h o f t h e a x e s o f i t s u n i t c e l l ; however, i n t h i s c a s e t h e χ
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In Chemistry of High-Temperature Superconductors; Nelson, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
4
CHEMISTRY
OF HIGH-TEMPERATURE
SUPERCONDUCTORS
c h a i n s do n o t i n t e r s e c t . Thus a general s t r u c t u r a l f e a t u r e of superconductors i s s t r o n g l y bonded c h a i n s t h a t are l i n e a r or n e a r l y so. T h e s e c h a i n s may o r may not intersect. In the case o f the t e t r a g o n a l K ^ N i F s t r u c t u r e , there are i n t e r s e c t i n g l i n e a r 4 N i - F f chains a l o n g two a x e s o n l y . In t h e c a s e o f t h e orthorhombic La Cu0 s t r u c t u r e , these chains are s l i g h t l y bent at the oxygen s i t e . For the R B a C u 0 s t r u c t u r e , t h e r e are i n f i n i t e l i n e a r non i n t e r s e c t i n g c h a i n s a l o n g one a x i s and t h e r e a r e n o n l i n e a r i n t e r s e c t i n g c h a i n s b u t o n l y a l o n g two a x e s . The i m p o r t a n c e o f t h e c h a i n s i n N b G e has b e e n d e b a t e d f o r many y e a r s , and u n d o u b t e d l y t h e i m p o r t a n c e o f c h a i n s i n t h e o x i d e s u p e r c o n d u c t o r s w i l l be d e b a t e d f o r y e a r s t o come. f
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CuoO and A g 0 a r e i s o s t r u c t u r a l and show t h e t e n d e n c y o f Cu* and A g to p r e f e r two-fold l i n e a r coordination to oxygen. The b a n d gap n a r r o w s on g o i n g f r o m C u 0 t o A g 0 and p r e s u m a b l y n a r r o w s s o much on g o i n g t o A u 0 t h a t t h i s compound d o e s n o t e x i s t . T h i s t r e n d f r o m Cu t o Ag t o Au c a n be t a k e n as a r e f l e c t i o n o f i n c r e a s e d c o v a l e n c y o f t h e m e t a l - o x y g e n bonds g o i n g down t h i s row o f t h e periodic table. CuO and AgO a r e a l s o w e l l known w h e r e a s AuO i s n o t . A l t h o u g h CuO c o n t a i n s C u , AgO c o n t a i n s a m i x t u r e o f A g and A g . T h i s tendency f o r the d i v a l e n t s t a t e t o d i s p r o p o r t i o n a t e i n c r e a s e s g o i n g down t h e p e r i o d i c t a b l e , Cu-*Ag->Au, t o s u c h an e x t e n t t h a t compounds w h i c h m i g h t appear t o c o n t a i n A u a c t u a l l y always c o n t a i n a m i x t u r e of A u and A u . On t h e o t h e r h a n d C u has n e v e r b e e n observed to d i s p r o p o r t i o n a t e i n oxide systems. I propose t h a t as one i n c r e a s e s t h e c o v a l e n c y o f t h e C u - 0 bond, Cu w i l l eventually disproportionate into Cu and Cu states. I f u r t h e r p r o p o s e t h a t t h e r e c a n be an i n t e r m e d i a t e r a n g e where C u in oxides e x h i b i t s d e l o c a l i z e d e l e c t r o n b e h a v i o r and s u p e r c o n d u c t i v i t y . I am t h u s s u g g e s t i n g a m e t a l l i c r e g i o n s a n d w i c h e d b e t w e e n two s e m i c o n d u c t i n g r e g i o n s ( F i g u r e 1 ) . The c o v a l e n c y o f C U - 0 bonds c a n be c o n t r o l l e d inductively. In s e r i e s s u c h as A C u O ^ , t h e c o v a l e n c y o f t h e Cu-0 b o n d s i n c r e a s e s as t h e A c a t i o n becomes more electropositive. Thus f o r a A-O-Cu l i n k a g e , Cu-0 bonds a r e f o r c e d more c o v a l e n t as t h e A-0 bond becomes more ionic. T h i s c a n be t h o u g h t o f as an i s s u e o f c o m p e t i t i o n between A and Cu f o r o v e r l a p o f t h e o x y g e n o r b i t a l s . P e r h a p s a b e t t e r v i e w i s t h a t when a h i g h l y e l e c t r o p o s i t i v e c a t i o n i o n i z e s , the e l e c t r o n d e n s i t y s h i f t s t o t h e r e s t o f t h e s y s t e m , e.g. o x y g e n and copper. When t h e Cu-0 bond i s i n t h e i o n i c l i m i t , t h e C u i o n w i l l o f c o u r s e e x h i b i t t h e b e h a v i o r o f a l o c a l i z e d dP ion. However, as t h e c o v a l e n c y i n c r e a s e s , t h e Cu-0 d i s t a n c e d e c r e a s e s , the m i x i n g w i t h oxygen o r b i t a l s 2
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In Chemistry of High-Temperature Superconductors; Nelson, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
1.
High-Temperature Superconductivity in Oxides
SLEIGHT
5
i n c r e a s e s and i n a c o n c e n t r a t e d s y s t e m t h e l e v e l s b r o a d e n i n t o bands w h i c h c a n s u p p o r t m e t a l l i c b e h a v i o r . The highest occupied state for C u i n the t e t r a g o n a l d i s t o r t e d o c t a h e d r o n ( d 2 - y 2 ) i s sigma a n t i b o n d i n g . The f a c t t h a t t h i s s t a t e i s s t r o n g l y a n t i b o n d i n g from symmetry c o n s i d e r a t i o n s i s o f l i t t l e c o n s e q u e n c e i n t h e ionic limit. However, as we i n c r e a s e t h e c o v a l e n c y , i t i s t h i s a n t i b o n d i n g s i t u a t i o n t h a t would cause C u to disproportionate into Cu and C u . Stabilization occurs f o r Cu b e c a u s e t h e 3d s h e l l f i l l s c o m p l e t e l y and thus s h r i n k s . S t a b i l i z a t i o n occurs f o r C u because the o f f e n d i n g σ* e l e c t r o n has disappeared. The π a n t i b o n d i n g s t a t e s f o r C u are a l s o f i l l e d , and t h e r e a r e no p o s s i b i l i t i e s f o r r e l i e f by depopulating 3d l e v e l s b e c a u s e t h a t w o u l d r e q u i r e a c o p p e r o x i d a t i o n s t a t e h i g h e r t h a n t h r e e w h i c h c a n n o t be a c h i e v e d . Again t h i s i s n o t an i s s u e i n t h e i o n i c l i m i t . However, as Cu-0 c o v a l e n c y i n c r e a s e s and Cu-0 d i s t a n c e s d e c r e a s e , t h e π* p r o b l e m i n c r e a s e s . The s y s t e m r e s p o n d s by b e n d i n g Cu-O-Cu bonds away f r o m 180° as i n L a C u 0 . This s t r u c t u r a l f e a t u r e i s a l s o present i n the R B a C u 0 _ phases. B e n d i n g t h i s bond away f r o m 180° i s c o u p l e d w i t h o x y g e n s-p r e h y b r i d i z a t i o n which pushes e l e c t r o n d e n s i t y o u t o f t h e bond. N a t u r e ' s way o f d e a l i n g w i t h a n t i b o n d i n g π s i t u a t i o n s i s t o b e n d bonds and convert a n t i b o n d i n g s t a t e s i n t o nonbonding s t a t e s . The r e a s o n t h a t Cu-O-Cu bonds b e n d i n L a C u 0 i s p r e c i s e l y t h e same r e a s o n t h a t C-O-C bonds a r e b e n t i n e t h e r s . Thus we f i n d l i n e a r Cu-O-Cu bonds i n t h e i o n i c l i m i t ( e . g . A C u 0 where A i s P r , Sm, Nd and Gd), b u t t h e bond b e n d s i n t h e c o v a l e n t l i m i t p r i o r to the d i s p r o p o r t i o n a t i o n l i m i t . 1 1
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The i n t e r m e d i a t e m e t a l l i c r e g i o n ( F i g u r e 1) i s n o t expected i n a l l Cu /0 systems. We c o u l d go d i r e c t l y from the i o n i c C u l i m i t to the c o v a l e n t C u + Cu limit. I n d e e d , t h i s i s t o be e x p e c t e d i n d i l u t e s y s t e m s where t h e r e a r e no Cu-O-Cu l i n k a g e s . In t h e L a _ A Cu0 s y s t e m s , t h e d i s p r o p o r t i o n a t i o n r e a c t i o n may be p r e v e n t e d by f r u s t r a t i o n . I t i s e a s y t o draw a L a C u 0 derived Cu-0 l a y e r w h i c h w o u l d c o n t a i n " i s o l a t e d " C u " 0 and linear Cu 0 units. However, s u c h o r d e r i n g m i g h t n o t be c o o p e r a t i v e f r o m one l a y e r t o t h e n e x t due t o s m a l l i n t e r l a y e r i n t e r a c t i o n s . More i m p o r t a n t l y t h e r e i s no good way t o s t a c k s u c h l a y e r s , and t h i s l e a d s t o f r u s t r a t i o n of the d i s p r o p o r t i o n a t i o n p r o c e s s . Furthermore, the s u b s t i t u t i o n of d i v a l e n t c a t i o n s leads t o d i s o r d e r on t h e A c a t i o n s i t e s as w e l l as e i t h e r oxygen v a c a n c i e s or s m a l l l e v e l s of C u . These d e f e c t s c o u l d a l s o i n h i b i t the d i s p r o p o r t i o n a t i o n r e a c t i o n which r e q u r e s long range c o o p e r a t i v e s t r u c t u r a l r e o r g a n i z a t i o n . I ] C
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Ba(Pb,Bi)Q^
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S i n c e many have c o n s i d e r e d s u p e r c o n d u c t i v i t y s y s t e m t o be t h e f i r s t example o f a new t y p e
in of
this
In Chemistry of High-Temperature Superconductors; Nelson, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
4
CHEMISTRY OF H I G H - T E M P E R A T U R E S U P E R C O N D U C T O R S
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s u p e r c o n d u c t i v i t y more r e c e n t l y d i s c o v e r e d i n m i x e d oxides of copper, i t i s important t o review the B a ( P b , B i ) 0 system f o r analogies t o the superconducting s y s t e m s b a s e d on c o p p e r o x i d e s . The d i s p r o p o r t i o n a t i o n reaction 2 B i i v ( g i ) _ > i " i (6s )+Bi (6s°) i s a c r u c i a l feature of the Β 3 Ρ ^ _ Β ι 0 system (Figure 2 ) . In t h e m e t a l l i c region, B i i s d i l u t e d i n a BaPb0 matrix. The 6s e l e c t r o n f r o m B i i s d e l o c a l i z e d i n a band d e r i v e d m o s t l y f r o m Pb 6 s and B i 6 s s t a t e s b u t w i t h s t r o n g a d m i x t u r e o f o x y g e n 2p s t a t e s . [Although paramagnetic s c a t i o n s such as B i a r e not observed i n concentrated systems, they a r e f r e q u e n t l y found i n d i l u t e systems such as g l a s s e s . ] As t h e B i c o n c e n t r a t i o n i n t h e Pb m a t r i x i n c r e a s e s , t h e B i atoms b e g i n t o s e n s e e a c h o t h e r a n d t h e tendency t o d i s p r o p o r t i o n a t e i n t o B i and B i increases. A t a b o u t 30% B i , t h e d i s p r o p o r t i o n a t i o n reaction occurs. The B i and B i do n o t t a k e o n l o n g range order a t t h i s p o i n t because they a r e s t i l l d i l u t e d i n a Pb m a t r i x . However, a s one a p p r o a c h e s p u r e B a B i 0 , B i and B i a r e o r d e r e d o n t o d i s c r e t e l a t t i c e s i t e s (8). Two o t h e r q u e s t i o n s s h o u l d be a d d r e s s e d i n t h i s system. Why i s B a P b 0 a m e t a l and why i s t h e s y s t e m s e m i c o n d u c t i n g above x—0.3? We know t h a t B a P b 0 i s b e s t v i e w e d a s a z e r o gap s e m i c o n d u c t o r , i . e . t h e c o n d u c t i o n and v a l e n c e bands j u s t s l i g h t l y o v e r l a p . By t h i s e x p l a n a t i o n we m i g h t e x p e c t t h a t Β 3 Ρ ^ « Β ΐ 0 c o m p o s i t i o n s s h o u l d remain m e t a l l i c even i f B i d i s p r o p o r t i o n a t e s t o remove B i 6 s s t a t e s f r o m E . However, a s χ i n c r e a s e s i n t h e Β 3 Ρ ^ _ Β ΐ 0 system, t h e l a t t i c e expands. Thus Pb-0 o v e r l a p d e c r e a s e s a n d a g a p opens up. 3
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3
Superconductivity
Superconductivity i n t h e B a P b ^ _ B i 0 s y s t e m and t h e r e c e n t l y d i s c o v e r e d copper oxide systems i s presumably r e l a t e d t o the disproportionation reactions of B i or Cu . In the B i s y s t e m , we a r e a b l e t o p u s h i n t o t h e r e g i o n where d i s p r o p o r t i o n a t i o n a c u t a l l y o c c u r s . One c o u l d s a y t h a t s u p e r c o n d u c t i v i t y o c c u r s when t h e situation 2 B i vs. Bi +Bi becomes d e g e n e r a t e . Iti s even tempting t o suggest t h a t lone p a i r s o f B i and C o o p e r p a i r s a r e somehow r e l a t e d . In the case o f copper o x i d e s u p e r c o n d u c t o r s , we c a n i n c r e a s e t h e c o v a l e n c y o f Cu-0 bonds enough t o p u s h i n t o t h e m e t a l l i c r e g i o n b u t we have n o t y e t i n c r e a s e d i t enough t o s e e t h e p r e d i c t e d disproportionation reaction. L a C u O ^ s i t s r i g h t o n t h e b o u n d a r y between t h e ^ i o n i c s e m i c o n d u c t i n g r e g i o n and t h e c o v a l e n t m e t a l l i c r e g i o n . As n o r m a l l y p r e p a r e d , L a C u 0 i s m e t a l l i c a t room t e m p e r a t u r e b u t becomes s e m i c o n d u c t i n g a t low temperature. S i n c e t h e r e h a s b e e n no symmetry c h a n g e , we know t h a t C u d i s p r o p o r t i o n a t i o n r e a c t i o n has not x
x
3
I
1 1
I
I V
V
i : c i
v
1
1
1
2
2
4
1 1
In Chemistry of High-Temperature Superconductors; Nelson, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
V
High-Temperature Superconductivity in Oxides
SLEIGHT
Disproportionation
Covalent
/ Ionic
Cu
Cu"-0~1.89Â
/
+ Cu
1
111
Downloaded by UNIV OF COLORADO AURORA on December 14, 2014 | http://pubs.acs.org Publication Date: August 28, 1987 | doi: 10.1021/bk-1987-0351.ch001
Semiconducting
/
Metallic
Cu -0~1.98Â 2+
Semiconducting
Τ
/Super-^Ny I conducting (La.Sr)
La Pr Nd Sm Eu Gd
1.08
1.10
1.20
Covalency of C u - 0 Bonds
