Chemical Aspects of High-Temperature Superconducting Oxides

Chapter 4 C h e m i c a l Aspects o f H i g h - T e m p e r a t u r e Superconducting Oxides 1

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Kazuo Fueki, Koichi Kitazawa, Kohji Kishio , Tetsuya Hasegawa, Shin-ichi Uchida , Hidenori Takagi, and Shoji Tanaka 2

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Department of Industrial Chemistry, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Department of Applied Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan

Downloaded by FUDAN UNIV on February 9, 2017 | http://pubs.acs.org Publication Date: August 28, 1987 | doi: 10.1021/bk-1987-0351.ch004

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The composition, structure and superconducting proper ties of high T oxide superconductors, La-Sr-Cu-O and Y-Ba-Cu-O systems have been investigated. From com parative studies of effects of partial replacements of metal sites by other elements, a low dimensional nature of conduction path was suggested in both oxide systems. Critical relevance of oxygen deficiency and the conditions of heat treatments to the evolution of superconductivity, is discussed in terms of the rela tively large nonstoichiometry found in these systems. c

Discovery of high T oxide superconductors was initiated by a pio neering work of Bednorz and Muller (l_-2) on a Ba-La-Cu-0 system. The succeeding reconfirmation by Uchida et al. (3) and the identifica tion of the superconducting phase as (La^_Ba)2Cu04 of the K2N1F4 structure by Takagi et al. (4) at the end of 1986 firmly established the record-breaking T of 30 Κ for the first time in 13 years. Intensive efforts were then initiated by many researchers to substi tute the component ions with similar elements on both of the La and Ba sites. The first success was reported by Kishio et al. (_5) with a (Lai__ Sr )2Cu04 system which exhibited T higher than the former by about 7 K. Other groups (6-7) independently reached the same mate rial shortly afterwards. Then the substitution on the La sites led Wu et al. (8) to find another superconductor, Y-Ba-Cu-0 system, and finally to overcome a long-waited technological barrier of liquid nitrogen temperature in February 1987. Independent discoveries of the same system were also reported (9-10) in a short period. The identification of the superconducting phase as Ba2YCu30y, a new crystal structure, was performed by Siegrist et al.(11) and further refined through neutron diffraction studies (12-13). The present paper is mainly concerned with chemical aspects of these oxide superconductors. By chemically modifying the composi tion, changes in structural parameters and superconducting proper ties of (Lai_ Sr )2Cu04 and Ba2YCu307 will be investigated in dec

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0097-6156/87/0351-0038$06.00/0 © 1987 American Chemical Society

Nelson et al.; Chemistry of High-Temperature Superconductors ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Chemical Aspects of Superconducting Oxides

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t a i l . This i s accomplished by p a r t i a l replacements of cation s i t e s by other elements, or by changing the oxygen stoichiometry through various heat treatment conditions. The effect of residual water on s u p e r c o n d u c t i v i t y i s a l s o presented. These r e s u l t s w i l l serve to c l a r i f y the e f f e c t s of i m p u r i t i e s and to show the n e c e s s i t y of p r e c i s e c o n t r o l of oxygen c o m p o s i t i o n i n o p t i m i z i n g the super conducting p r o p e r t i e s of these oxides. From a p h y s i c a l p o i n t of view, a low dimensional nature of conduction paths inferred from the present e x p e r i m e n t a l r e s u l t s and the c o m p a t i b i l i t y of supercon d u c t i v i t y with magnetic impurities are discussed. These information should be q u i t e h e l p f u l i n understanding the superconductivity of oxide superconductors realized at these e x t r a o r d i n a r i l y high tem peratures.


Effects of p a r t i a l replacements of component cations It i s now well known that a l l systems of (Lai_ M )2Cu04 where M=Ba, Sr or Ca at χ = about 0.1 are high-T superconductors with the same c r y s t a l structure, the tetragonal K2NiF4~type. After the d i s covery of these three systems, i t became of immediate i n t e r e s t whether these compounds were s o l u b l e to each other and how the s u p e r c o n d u c t i v i t y would be a f f e c t e d by mixing of a l k a l i n e e a r t h elements. A quasi-ternary composition of (Lai_ (Ba,Sr,Ca) )2Cu04 at x=0.1 was thus prepared and i t was shown that the entire composition was a homogeneous s o l i d solution and actually superconducting (14). Moreover, T was found to vary smoothly w i t h composition. The maximum T was found to be 37.0 Κ and a minimum, 18.2 K, both of which were found at corner compositions of Sr and Ca, respectively. In order to f i n d out f a c t o r s which determine the superconducting properties, the tetragonal l a t t i c e parameters, ag and eg, of various compositions are p l o t t e d , i n F i g . 1, as a f u n c t i o n of onset T determined from a.c. s u s c e p t i b i l i t y measurements. I t i s immediately noticed that ag i s rather well correlated with T , while CQ i s not. Close check of composition reveale that CQ seems to r e f l e c t simply the difference i n the ionic sizes of a l k a l i n e earth elements. On the other hand, the strong correlation of ag and T was also supported by high pressure studies of the Sr system (15-16). Replacement of Ba by Sr i n the case of the La-Ba-Cu-0 system was thus favorable i n decreasing ag and increasing T . However the same i s not t r u e i n the case of the Y-Ba-Cu-0 system. Figure 2 shows the orthorhombic l a t t i c e parameters of ag, bg and eg for (Baj_ Sr )2YCu307 for x=0 to 0.7. It i s c l e a r l y seen that these parame ters a l l decrease with Sr composition, x. Figure 3 shows the r e s i s t i v i t y vs. temperature of these specimens. Quite unexpectedly from the d i s c u s s i o n s up to t h i s p o i n t , T i n t h i s p a r t i c u l a r system decreases with increase i n x. Although the difference i n these two examples i s not c l e a r yet, i t should be q u i t e e s s e n t i a l i n under standing the mechanism of the superconductivity i n the both systems. P a r t i a l replacements of the r a r e e a r t h s i t e s were a l s o exa mined. F i g u r e 4 shows the e f f e c t of 5% s u b s t i t u t i o n of 4f i o n s on La s i t e s for the La-Sr-Cu-0 system (17). The l a t t i c e parameters and T are p l o t t e d i n the order of atomic number. Since most of these ions possess unclosed 4f o r b i t a l s , they are considered to be magne t i c impurities. It i s interesting to see that T f i r s t decreases, passes through a minimum for Gd and increases again. This suggests x

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Fig. 1 : Tetragonal l a t t i c e parameters as a function of onset T (ac s u s c e p t i b i l i t y ) for s o l i d solution system (Lao.9(Ba,Sr,Ca)o,i)2 4« Indices attached to each data point express the r a t i o of a l k a l i n e earth ion compositions i n the order of Ba, Sr and Ca; e.g. [013] represents ( L a o . 9 S r . 0 2 5 0 . 0 7 5 ) 2 4 ' Reproduced with permission from Ref. 14. Copyright 1987, The Chemical Society of Japan. c

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FUEKI ET AL.

Fig. 2 : L a t t i c e parameters of (Bai_ Sr )2YCu30y as a function of x. x

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Fig. 3 : R e s i s t i v i t y vs. temperature of (Bai_ Sr )2YCu307. x

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interactions of conduction electrons with the 4f s h e l l spins on the La s i t e , although the e f f e c t should be considered to be quite small i n terms of the destruction of the superconductivity. In the case of the Y-Ba-Cu-0 system, 4f magnetic ions may not be c o n s i d e r e d to be i m p u r i t i e s as f a r as the s u p e r c o n d u c t i v i t y i s concerned. As many i n v e s t i g a t o r s have discovered(18-24), most of the rare earth elements give 90 Κ superconductors without any appre ciable difference i n T . K i s h i o et a l . (25) have shown that 90 Κ conductors can be a l s o s u c c e s s f u l l y prepared even from c e r t a i n c o m p o s i t i o n s of unseparated mixture of r a r e e a r t h s as s t a r t i n g materials. Although the replacements of La or Y s i t e s by 4f magnetic ions have been shown to be rather i n s e n s i t i v e to superconducting proper t i e s of both La-Sr-Cu-0 and Y-Ba-Cu-0 systems, s i m i l a r replacements of Cu s i t e s are found to be more s i g n i f i c a n t . Hasegawa et a l . (26) have shown that i n c o r p o r a t i o n of 3d elements, or Zn on the Cu s i t e of La-Sr-Cu-0 at the l e v e l of 5 atomic percent was enough to nearly destroy the superconductivity. Figure 5 shows the effect of repla cement of Cu w i t h Fe i n the Y-Ba-Cu-0 system. In t h i s case, i t i s seen that 3% doping reduces T to almost a half value and 10% doping loses z e r o - r e s i s t i v i t y state. A l l of the e f f e c t s of replacements of each metal s i t e so f a r described can be understood within a framework of a low dimensional nature of conduction mechanisms. More s p e c i f i c a l l y , superconduction takes p l a c e through the O-Cu-0 bonds of the i n t e r c o n n e c t e d CUO5 octahedra or CUO4 squares i n the La-Sr-Cu-0 and Y-Ba-Cu-0 systems, respectively. The La or Y s i t e s are less sensitive f o r the presence of magnetic impurities and seem to be rather away from the super conduction path. This tendency i s more s i g n i f i c a n t i n the Y-Ba-Cu-0 system, because the conduction path would be along the one dimensio nal chains of O-Cu-0 bonds which are most apart from the Y s i t e s .


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Oxygen nonstoichiometry and significance of heat treatment It has been well recognized that the superconducting properties of both La-Sr-Cu-0 and Y-Ba-Cu-0 systems are quite sensitive to the oxygen composition which i s e a s i l y altered by various heat t r e a t ments. Examples are shown i n F i g s . 6 and 7. F i g . 6 shows the e f f e c t of post-annealing on the r e s i s t i v i t y of (Lao.92Sro.08)2^ ^4* As i s seen , the specimen prepared at 1100°C and q u i c k l y cooled shows a c l e a r onset at about 38 Κ but i t e x h i b i t s a s i g n i f i c a n t l y broad t r a n s i t i o n and a t a i l i n g structure supressing the zero r e s i s t i v i t y to as low as 24 K. This t a i l i n g structure i s presumed to be due to formation of oxygen depleted region i n the v i c i n i t y of grain boundaries of the p o l y c r y s t a l l i n e s t r u c t u r e formed d u r i n g the cooling process. When the specimens are annealed i n oxygen at lower temperatures, the t r a n s i t i o n becomes a p p r e c i a b l y sharper and the r e s i s t i v i t y decreases. The best r e s u l t has been obtained w i t h annealing at 600°C for 50 h, while lower temperatures seem to be too low i n s u f f i c i e n t incorporation of oxygen within a l i m i t e d time of heat treatment. Figure 7 presents the s i m i l a r e f f e c t of heat t r e a t ment i n the Y-Ba-Cu-0 system. Specimens of Ba2YCu30y were quickly cooled from various temperatures a f t e r annealing i n pure O 2 . I t i s seen i n Fig. 7 that the e f f e c t of annealing i s much more pronounced i n the Y-Ba-Cu-0 system. u



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Fig. 4 : Variations of tetragonal l a t t i c e parameters ag and CQ and T for the s o l i d solution systems of (Lao 85Lno.05Sro.l)2Cu04 with d i f f e r e n t l a n t h a n i d e ions. Reproduced with permission from Ref. 17. Copyright 1987, Publication Board, Japanese Journal of Applied Physics. c

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Fig. 5 : R e s i s t i v i t y vs. temperature of Ba2YCu3(i_ )Fe3 07. x


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F i g . 7 : Effect of O2 annealing i n Ba2YCu30y at various temperatures.


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Since these remarkable changes i n superconducting behaviors of h i g h - T oxides are a s s o c i a t e d w i t h the oxygen c o m p o s i t i o n of the specimens, i t i s essential to determine equilibrium values of oxygen composition under various conditions. In t h i s paper we describe the r e s u l t of oxygen nonstoichiometry measurements of the I ^ Y C ^ O y ^ . The measurement was made by a thermogravimetric technique with the aid of chemical analysis using iodometric t i t r a t i o n to determine the a b s o l u t e v a l u e s of oxygen compositions. As d e t a i l s are found elsewhere (27), only the observed r e s u l t i s presented i n Fig. 8. The n o n s t o i c h i o m e t r y i s found to vary c o n t i n u o u s l y as a f u n c t i o n of temperature and oxygen p a r t i a l pressure. The maximum deviation from the tentatively assigned stoichiometric composition 7 i s about -0.9 amounting to 13% of the t o t a l oxygen content, suggesting the nonstoichiometry i s exceptionally large i n Y-Ba-Cu-0 system. The r e s u l t suggests that the annealing i n an appropriate atmosphere and at r e l a t i v e l y low temperatures below 500°C i s important to minimize the oxygen deficiency. However, prolonged annealing i n the oxygen atmosphere may not be n e c e s s a r i l y optimum, a c c o r d i n g to our p r e l i m i n a r y r e s u l t s . Appearance of h i g h l y r e s i s t i v e g r a i n boundary l a y e r has been suggested to form, i f the specimen i s subjected to prolonged annealing at low temperatures. T h i s i n d i c a t e s the n e c e s s i t y of c a r e f u l l y designed annealing program for the p r a c t i c a l f a b r i c a t i o n process of the oxide superconducting materials.


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E f f e c t of water i n superconductivity i n La-Sr-Cu-0 system In addition to m e t a l l i c ions, some other impurities may a f f e c t the superconductivity of the high-T oxide superconductors. Water was found to be one of such s p e c i e s i n the e a r l y stage of our i n v e s t i g a t i o n . F i g u r e 9 shows an e f f e c t of r e s i d u a l water(28) on the volume f r a c t i o n of s u p e r c o n d u c t i v i t y i n ( L a i _ S r ) 2 C u 0 4 at x=0.08. The samples were prepared i n presence of water i n t e n t i o n a l l y added d u r i n g the mixing procedure of s t a r t i n g m a t e r i a l s . The samples had been f i r e d at 1100°C for 22 h. As the expected amounts of residual water i n the samples A to E decrease, the volume f r a c tions are seen to increase i n Fig. 9. The r e s i s t i v i t y measurements of these samples exhibited almost same T but the temperature dependence of the r e s i s t i v i t y (28) was w e l l c o r r e l a t e d again w i t h the amount of residual water. Although X-ray d i f f r a c t i o n analyses could not detect formation of any extra phase i n these samples, the e f f e c t of r e s i d u a l water i n d e t e r i o r a t i n g the superconductivity i s quite clear i n t h i s oxide. c

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Conclusion Some c h e m i c a l a s p e c t s such as the e f f e c t s of c o m o s i t i o n and i m p u r i t i e s on the m a n i f e s t a t i o n of s u p e r c o n d u c t i v i t y i n h i g h - T oxides have been described. They are shown to be quite s i g n i f i c a n t i n d e t e r m i n i n g superconducting behavior and probably many other physical properties of these materials as well. As the developments of oxide superconductors with higher T and better quality continue, i t should be essential to take these aspects f u l l y into account. c

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Nonstoichiometry of Ba2YCu3U7_ g as a function of oxygen p a r t i a l pressure. Reproduced with permission from Ref. 27. Copyright 1987, Publication Board, Japanese Journal of Applied Physics.

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Chemical Aspects of High-Temperature Superconducting Oxides

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