Physica C 185-189 (1991) 593-594 North-Holland

II I I I I I

SYNTHESIS OF SUPERCONDUCTING OXIDES IN THE SYSTEM OF G d . - L a - C e - C u - O

Yuichl WATANABE, Yoshio MIZUTANI, Masasuke TAKATA, Mamoru YOSHIMOTO* and Hideomi KOINUMA*

Department of Electrical Engineering, Nagaoka University of Technology, Naqanka, Niigata 940-21 JAPAN / *Research Laboratory u2 Engineering Materials, Tokyo Institute of Technology, Midori-ku, Yokohama, Kanagawa 227 JAPAN

Superconduct ing ox ides in the system of Gd-La-Ce-Cu-O have been s y n t h e s i z e d f o r t h e first time on the basis of structural prediction from the tolerance factor. The To,onset was found to be 5-II.6K for the samples of Gd2.z.yLayCezCuO4_6with compositions x=0.1-0.2 and Gd:La:l:2. The negative signs of Seebeck coefficients indicated that the system might belong to n-type superconducting oxides such as Nd-Ce-Cu-O system.

i. Introduction

At present, the electron-doped superconduc-

tors have been synthesized only for the system

Ln2_xCexCuO4_ b in which Nd, Pr, Sm, and Eu

were used as Ln 1,2. The systems were charac-

terized by the T' structure in which lantha-

hide elements are surrounded by 8 neighboring

oxygen atoms. Thereby, ionic radius for

lanthanide elements seems to be essential for

the formation of T' structure. Then, it seems

to be meaningful to further examine the adapt-

ability of the "size effect" on the formation

of T' structure. One of the most advanta-

geous factors for prediction of resulted

structure for a given system is the "toler-

ance factor "3 .

In the present paper, we report on the

formation of T'- structure showing supercon-

ductivity with negative Seebeck coefficient

for the system Gd2_x_yLayCexCuO 4_ b • It should

be noted that both the element Gd and La have

been confirmed not to form superconduc[ors

with T; structure when used singly. ?hough,

it will be evidenced that the precise selec-

tion of tolerance factor has a great advantage

in material design with respect to the forma-

tion of superconducting T'-structure.

2. EXPERIMENTAL PROCEDURES

The Gd2_x_yLayCexCuO 4_b samples were pre-

pared by conventional solid-state reaction

method. Gd203 (purity 99.99%), La203(purity

99.99%), CeO2(purity 99.99%) mud CuO(purity

99.9%) were used as raw materials. The pel-

leted mixture of desired composition was sin-

tered at I040°C for 8h in air and cooled to

room temperature in an electric furnace.

Additional annealing treatments were carried

out for some of the obtained samples at

800*C-1000°C for 8h-91h in Ar atmosphere and

subsequently quenched by immersing the an-

nealed pellet into liquid nitrogen.

3. RESULTS AND DISCUSSION

Crystal structures for the samples were

assigned to be T'-phase which had beer. expect-

ed from the preliminary compositional control

O (Gdl/3La2 / 3 ) 2-XCexCuO4 {

22 X Nd2.xCexCuO4

- L

~ 20

I 4 Io

;

Z 0 ~ - " - r - - I - - - . " Q . , : ' I ,

0 0.05 0.10 0.15 0.20 0.2~ 0.20 x ICe )

Fig.l Changes in Tc,onse t ~s a function of Ce

content x in (Gdl/3La2:3)2-xCexCu04-'-.

0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B V. All rights re.~crved.

594 Y Watanabe et al. / Superco.ducting oxides in the system of Gd-La.Ce.¢u-O

of a v e r a g e i o n i c r a d i u s be tween Gd 3+, La 3+,

and Ce 4+ so as to have the tolerance factor

ranged from 0.850-0.856.

Figure 1 shows the re la t ionsh ip between

Tc,onse t and Ce content x fo r the system

(Gdl/3La2/3)2_xCexCuO4_6. As seen in the

f igure, Tc,onse t fo r the present system can

only be observed in the spec i f i c range of Ce

content, x=O.l-0.2. Though the Tc,onse t were

found to be suppressed below 9.5K, there seems

to remain a p o s s i b i l i t y for fu r the r increase

in the Tc,onse t by optimizing the concentra-

tion of oxygen deficiency, Ce content, and Gd-

La ratio. It should be noted that the region

of Ce content at which the system shows super-

conductivity is wider than that for Nu-Ce-Cu-O

system (x=0.14-0.17) 4.

Figure 2 shows the lattice parameters of

the T'-phase of (Gdl/3La2/3)2-xCexCu04- 6 . As

seen in the figure, a-axis of the T'-phase is

increased with increase in Ce content, while

the c-axis shows maximum at the Ce content of

about x=0.15. It should be noted that in Nd

system, c-axis of the T'-phase was found to be

reduced with increase in Ce content followed

by the size effect; Ce substituted for Nd site

has smaller ionic radius than that of Nd. The

present result can also be interpreted in

terms of the size effect. Firstly, the short-

ening of c-axis at above x=0.17 in Fig.2 may

be due to the ordinary size effect as well as

that for Nd system, since only T'-phase was

observed for this Ce content region. The

phase singularity indicates the high ability

for the formation of solid solution among Ce,

Gd, and La within x~0.17. On the other hand,

the shortening of c-axis with decrease in Ce

content below x=0.17 should be considered

together with the fact that significant amount

of T-phase was formed correspondingly with

decrease in Ce content. The formation of T-

phase results in the decrease in average ionic

radius among remained La, Gd, and Ce, which

3.98

3.97

3.98

3.95

0 0

i i I I i

850t-~

Q

O 0

0

O0 0

12.26

12.24

12.22 o<

12.20 ¢)

12.19

12.18

12.1¢ I I ,, ,I I I I I

0 0.05 0.10 0.15 0.20 0.25 0.30 Ce CONTENT (x)

Fig.2 Relationship between Ce contents x in

(Gdl/3La2/3)2-xCexCu04-6 and lattice parame-

ters, a- and c-axes.

will be consumed for the formation of T'

phase. Therefore, the c-axis of resulted T'-

phase becomes short with increase in the

amount of T-phase. Consider that the super-

conducting transitions were observed only for

the samples with the Ce content of x=0.1-0.2,

it can be concluded that the appearance of

superconductive nature in the present system

is closely related not only to the formation

of T'-phase but also to the lattice parameter,

especially the length of c-axis.

The wider range of Ce content for the

superconductive nature of the present system

may be due to the effect of mixing of lantha-

nide elements by which the system has a compo-

sitional flexibility to form solid solution of

T' phase with suitable c-axis among lanthanide

elements and Ce.

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