A systematic study of Bi2−xPbxSr2Ca2Cu3Oy phases (0≤ x ≤ 2)

Mat. R e s . B u l l . , Vol. 25, p p . 223-229, 1990. P r i n t e d in t h e USA. 0025-5408/90 $3.00 + .00 C o p y r i g h t (c) 1990 P e r g a m o n P r e s s p l c .

A SYSTEMATIC

STUDY

OF

Bi

2-x

Pb Sr Ca Cu O x

2

2

3

y

PHASES

(0~ x ~ 2)

Seharis-Rodriguez, F. G a r c i a - A l v a r a d o , E. Moran, Alario-Franco (*), M. A . G o n z a l e z , J. L. V i c e n t (**), and J . I b a ~ e z ( * * * ) . (*) Dpto. Q u i m i c a Inorganica, Fac. de C i e n c i a s Quimicas, U n i v e r s i d a d C o m p l u t e n s e . 28040 Madrid. S p a i n ; (**) Dpto. de F i s i c a de la M a t e r i a Condensada, Fac. de C. Fisicas, Univ. C o m p l u t e n s e . 28040 Madrid. S p a i n ; (***) C.E.N.I.M, C.S.I.C, 28040 Madrid. Spain. M.A.

M.A.

( R e c e i v e d O c t o b e r 20, 1989; C o m m u n i c a t e d b y P. H a g e n m u l l e r )

ABSTRACT By m e a n s of X - r a y d i f f r a c t i o n , e l e c t r o n d i f f r a c t i o n and scanning electron microscopy, we h a v e o b s e r v e d t h a t for low Pb/Bi r a t i o s lead i n c o r p o r a t e s into the s t r u c t u r e of the s o - c a l l e d 80 K and Ii0 K B i - s u p e r c o n d u c t o r s , which n o r m a l l y coexist. H o w e v e r , t h e 2223 s u p e r c o n d u c t i n g phase, w h i c h shows zero r e s i s t a n c e at a t e m p e r a t u r e as h i g h as 110.7 K , becomes predominant as the sintering time increases. F r o m x=0.5 u p w a r d s an a l m o s t B i - f r e e s e c o n d phase appears. It is non-superconducting and its s t r u c t u r e a p p e a r s to be hexagonal. S o m e C a z P b ~ is also p r e s e n t for x > 0.5. M A T E R I A L S INDEX: oxides, bismuth, strontium, calcium, copper, lead, superconductors Introduction Since the discovery of h i g h Tc superconductivity in the B i - S r - C a - C u - O s y s t e m (i), m a n y d i f f i c u l t i e s h a v e b e e n e x p e r i e n c e d in t r y i n g to p r o d u c e a m o n o p h a s i c 2223 compound, w h i c h shows the h i g h e s t Tc so far o b s e r v e d in this system. For t h i s reason, s e v e r a l p r o c e d u r e s h a v e b e e n a p p l i e d to i n c r e a s e the v o l u m e fraction of t h a t phase: annealing at v e r y p r e c i s e temperatures (2), c h o i c e of some p a r t i c u l a r s t a r t i n g c o m p o s i t i o n s (3,4) , l o n g f i r i n g p e r i o d s (5), etc . . . . and Lead a d d i t i o n (6) - t h i s last b e i n g the m o s t e f f i c i e n t one. 223

224

M . A . S E N A R I S - R O D R I G U E Z , et al.

Vol. 25, No. 2

In t h i s c o n n e c t i o n , w e h a v e c a r r i e d out a s y s t e m a t i c s t u d y of t h e Bi2. x P b x S r z C a z C u 3 O y s y s t e m in o r d e r to i n v e s t i g a t e h o w Lead d i s t r i b u t e s into the d i f f e r e n t p h a s e s present, as w e l l as t h e i r range of existence and its effect on the superconducting properties It is w o r t h n o t i n g that, as we will show below, even when sharp r e s i s t i v e t r a n s i t i o n s w e r e observed, electron diffraction (ED) and e l e c t r o n microscopy (SEM and TEM) reveal that the s a m p l e s are i n h o m o g e n e o u s .

Experimental S a m p l e s w i t h n o m i n a l c o m p o s i t i o n • Bi 20-- , P b ~_ S r 2 C a 2 C u 3 0 0, 0 ° s x ~ • 2 w e r e p r e p a r e d by s o l i d state r e a c t l o n in alr. T h e s t o l c h l o m e t r l c a m o u n t s of Bi203, PbO, SrCO3, CaCO 3 and CuO (Merck A R grade) w e r e mixed, pressed into p e l l e t s and fired in g o l d crucibles at 860 °- 8 7 0 ° C f o r 30-55 h o u r s w i t h i n t e r m e d i a t e grindings. P o w d e r d i f f r a c t i o n p a t t e r n s of the s a m p l e s w e r e o b t a i n e d w i t h a S i e m e n s D-500 apparatus. E l e c t r o n d i f f r a c t i o n w a s d o n e on a J e o l 2000 FX instrument. S t a n d a r d DC four p r o b e t e c h n i q u e w a s used for resistance measurements. Magnetic susceptibility measurements were made using an A S M I pendule magnetometer. Composition analysis was c a r r i e d out by m e a n s of a J e o l 840 s c a n n i n g e l e c t r o n m i c r o s c o p e (SEM), w i t h a L i n k e n e r g y d i s p e r s i v e s p e c t o m e t e r (EDS).

Results

and d i s c u s s i o n

A l t h o u g h the s y n t h e s i s of some s u p e r c o n d u c t i n g p h a s e s in the B i - P b - S r - C a - C u - 0 s y s t e m does not s e e m to be too c o m p l i c a t e d , m a n y different phases may coexist, so that the study of the microstructure of t h e s e m a t e r i a l s is e s s e n t i a l to u n d e r s t a n d their properties. F i g u r e s la and ib, c o r r e s p o n d i n g to the s a m p l e s w i t h x=0.5 t r e a t e d at 860 ° C for d i f f e r e n t l e n g t h s of t i m e (55 and 210 h o u r s respectively) s h o w t h a t b o t h s a m p l e s are h e t e r o g e n o u s . The composition of the p r e d o m i n a n t phases in some of the samples, as o b s e r v e d from EDS analysis, are shown in t a b l e I. By means of X - r a y diffraction phases "A" a n d "C" w e r e clearly identified as their, d o o 2 spacings are 15.6 A and 18.9 A r e s p e c t i v e l y (figs. 2a and 2b).

Vol. 25, No. 2

SUPERCONDUCTORS

225

FIG. 1 Backscattered electron micrographs of polished samples with x=0.5 heated at 860°C 55 hours (a) and 210 hours (b). The microstructure consists of unreacted copper oxide (black regions), the "A" and "B" phases (ligth areas) and the "C" phase (dark areas).

226

M.A. SEI~ARIS-RODRIGUEZ, et al.

Vol. 25, No. 2

TABLE I Results from EDS Analysis in some Bi2_xPbxSr2Ca2Cu3OySamples. X

A

B

C

Q.3

Bil.74Po0.385ci.48Cai.01CU20y Bio.34Pb1.975r2.17Ca 1.75CUOy 8ii.75Pbo.455ri.70Cai.80CU30y

0.5

BiI.59Pbo.535rI.53Cao.97CU#y Bio.16Pb2.30Sr2.19Ca 1.7#UOy

I

Bi1.61Pbo.505ri.52CaO.91CU20y Bio.23Pbz.o65r1.98Cai .71CUOy

2

Bil.73Pbo.505ri.70Cai.80CU30y

?b2.51Sr2.21Cal.58Cd3y

21

8 6 o ° c 1 2 1 0 hrs.

,q

4

gD

=

i

J 20

35

~,

35

i 5

860°CI 55

20

rs.

i

5

--. 2 e ( K = Cu) FIG. 2 X-ray diffraction patterns of samples with nominal composition Bil.sPb~rzCazCu3Oy. Both, the two-CuO layer phase (o) and the three- Cu0 layer one (A) coexist, but long heating times clearly favour the formation of the Bi-phase with c=37A.

Vol. 25, No. 2

SUPERCONDUCTORS

227

It can be o b s e r v e d (table I) that, for low Pb/Bi ratios, Lead i n c o r p o r a t e s into the s t r u c t u r e of b o t h the t w o - C u O l a y e r and the three-CuO layer Bi-superconductors ("A" and "C" p h a s e s in t a b l e I, r e s p e c t i v e l y ) . For x