Effect of Cu substitution by Zn Cd and Hg upon superconductivity in YBa2Cu3O7 and La1.85Sr0.15CuO4

Physica C 153-155 (1988) 906-907 North-Holland, Amsterdam

EFFECT OF YBa2Cu307

Cu SUBSTITUTION BY Zn Cd AND Lal.85Sro. 15CuO 4

K. R E M S C H N I G * a n d P. R O G L * R. EIBLER, G . H I L S C H E R , N. P I L L M A Y R ,

AND

Hg

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H. K I R C H M A Y R

SUPERCONDUCTIVITY

and

IN

E. B A U E R

Xlnst.f. Physikalische C h e m i c Univ. Wien, W a h r i n s e r s t r . 4 2 Inst.f. E x p e r i m e n t a l p h y s i k T.U. Wien, W i e d n e r H a u p t s t r . 8 ,

, A-lOgO Wlen A-I040 Wien

We r e p o r t on t h e structural c h e m i s t r y , ac-susceptibilltyand s p e c i f i c heat measurements o~ doped ceramic superconductors. In b o t h s y s t e m s a s p e c i f i c h e a t linear term Z T is r e s o l v e d in t h e s u p e r c o n d u c t i n g state which increases substantially upon Cu/Zn substitution, w h i l e Tc is r e d u c e d .

i.

INTRODUCTION The discovery of hlgh-Tc superconductivity in t h e ceramic Cu-oxides ( L a B a C u O (I,2) a n d Y B a C u O (3)) g e n e r a t e d considerable effort in characterizing their physical properties. In t h i s p a p e r we b r i e f l y describe s o m e of o u r r e c e n t experiments on t h e effect of chemical substitution on the structural and superconducting properties of these materials.

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METHODS AND MATERIALS Sample preparation has recently been described in detail (4,5). Lattice parameters and standard deviations were evaluated by a l e a s t s q u a r e s refinement of r o o m temperature Guinier-Huber X-ray powder photographs. Ac-susceptibility measurements were performed with a standard lock-in technique at 8 1 H z w i t h a field amplitude of 1 0 e r s t e d . The heat c a p a c i t y of s a m p l e s w i t h a m a s s of a b o u t 2g was measured over the temperature range 1.5 60K in an automated adiabatic calorimeter. 3.

RESULTS AND DISCUSSION W h i l e Z n w a s f o u n d to s t a t i s t i c a l l y r e p l a c e C u to a r a t h e r large extent in Lal. esSro, ls(Cul-xZn×)04 (0 ~ x S 0.45; IIO0°C, quenched>, solid solutions of YBaz(Cui-×Zn×)sO?-6 were observed to be limited to O~x
0921-4534/88/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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FIGURE 1 Lattice parameters as a function of Cd and Hg containing samples appear be m u l t i p h a s e f o r x > 0,02.

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also known for the low temperature form of LazCu04 and Lal. ssSro.imCu04. The variation of t h e u n i t c e l l d i m e n s i o n s as a function of the Cu/Zn-ratio is reflected by a corresponding chanse in the tstragonal bipyramldal oxygen coordination of t h e C u ( Z n ) - a t o r a s t o w a r d s a more regular ootahedron at higher Zn-content. Accordingly the c/a ratio decreases well below the rather high value of 3.5, which was claimed t o be indicative of a J a h n - T e l l e r effect on the c e n t r a l C u - a t o m .

K. Remschnig et at / Effect of Cu substitution by Zn, Cd and Hg

Irrespective of s a m p l e t r e a t m e n t O.02. In o r d e r to s t u d y t h e i n f l u e n c e of s i z e e f f e c t s a n d / o r v a l e n c e electron concentration the Isoelectronic substitution of C u by p s e u d o e l e m e n t a l Cu i.e. (Nio.sZno. s) w a s i n v e s t i g a t e d . The degradation of s u p e r c o n d u c t i v i t y upon Cd and Hg substitution is f o u n d t o be f a i r l y s m a l l f o r b o t h the Y- a n d L a system (on a v e r a g e 1,5 K / m o l t C d or Hg) in c o m p a r i s o n with Cu/Zn substitution: t h e i n i t i a l s l o p e is 8 . 5 K / m o l t a n d 11,5 K/molt Zn for YBaCuO and LaSrCuO respectively, The onset of the superconducting transition Tco r e m a i n s within 0.3K unchanged whilst the m i d p o i n t of t h e t r a n s i t i o n (Tom t a k e n at 50%) shifts to lower temperatures and the transition width (ATe i0%-g0%) increases upon Hg substitution in t h e Y - s y s t e m f r o m 2 , 3 K to 16K for x = O . O a n d x=O.05 respectively, The same tendency is observed in the La-system, even t h o u g h o n l y f o r 1% C d or Hg, b e c a u s e of the limited solid solution range mentioned above. In c o n t r a s t to Cd and Hg substitution Cu/Zn and isoelectronic C u / ( N i o . ~ Z n o , 5) r e p l a c e m e n t shifts both Teo a n d Tom w i t h a m u c h l a r g e r v a r i a t i o n to lower temperatures. Cu/Zn-substitution appears to have a much larger effect than other 3d-metals <6,7). The high-Tc ceramic Cu-oxide superconductors s e e m to be c h a r a c t e r i z e d by a finite Z T contribution to the s p e c i f i c h e a t in the s u p e r c o n d u c t i n g state. This 2 t e r m in u n i t s of m J / C u - m o l K increases dramatically upon Cu/Zn substitution in both high-To systems: ~ rises from 2,5 at x=O.O to 10.4 at x=0.025 in Lai. ssSro, ls(Cui-×Z~×)04, and the variation of Z in YBa2
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FIGURE 2 Supercond, trans, temp. Tc a s a f u n c t i o n of Cu/X substitution as labeled, and isoelectronic (Nio,sZno.s) substitution (*). Tco= Tc onset, Tom= Tc m i d p o i n t . these materials, and may arise from unpaired carriers far below Tc d u e to r e g i o n s on t h e F e r m i s u r f a c e over which the energy gap vanishes. In t e r m s of this speculative view the dramatic increase of ~ upon Cu/~n substitution in both systems can reasonably be explained by a growing extent of non-superconducting carriers whereby Tc a n d ACp/Tc (5) d e c r e a s e s . REFERENCES and K,A, M u l l e r , Z, (I) J.G, B e d n o r z Phys. B 6 4 (1986) 189 R , J . C a v a , R . B . v a n D o v e r et al. <2) Phys. Rev, Lett. 5 8 (1987) 4 0 8 M.K.Wu, J.R.Ashburn et al, (3) Phys. Rev. Left. 5 8 <1987) 9 0 8 K. R e m s o h n i g , P , R o g l , E, B a u e r et al. (4) Proc. Int. D i s c u s s i o n Meeting Mauterndorf Feb. 1988, P l e n u m P r e s s G. H i l s c h e r , N.Pillmayr, R.Eibler (5) et al, , s u b m i t t e d to Z. Phys, G,Xiao, F.H. S t r e i t z , A , G a r v i n et (6) al, ,Phys. Rev. B 3 5 (1987) 8 7 7 2 <7) S.B.Oseroff, D,C. Vier, et al. Sol.grate Comm. 8 4 (198?) 2 4 1 A, Junod, A. B e n z i g e et al, ,Jap. J. <8) Appl, Phys. ~ 6 <1987) 1 1 1 g M.B, M a p l e Proc. [nt D i s c u s s , M e e t l n g <9) Mauterndorf Feb. 1 9 8 8 , P l e n u m Press