Physica C 235-240 (1994) 537-538

PHYSICA

Mercury and thallium based superconducting oxycarbonates

C. Michel, M. Huv6, C. Martin, A. Maignan, D. Pelloquin, F. Goutenoire, M. Hervieu and B. Raveau

Laboratoire CRISMAT - CNRS URA 1318 -ISMRA - Universit6 de CAEN 6 Boulevard du Mar6chal Juin, 14050 CAEN Cedex FRANCE

Five layered oxycarbonates involving mercury or thallium have been synthesized and characterized on a structural point of view and on that of the physical properties. Critical temperatures up to 70 K and superconducting volume fractions higher than 50 % can be achieved.

1. INTRODUCTION

The ability of carbonate groups to accomodate the perovskite framework [1] has been demonstrated with the synthesis of Sr2CuO2CO3.. Two types of ordered phases related to the perovskite have then been isolated : A CUl+y(CO3)l.xO2 (A = Ba, Sr) with a maximum Tc of 40 K [2] and the "123" derivative oxycarbonate with a Tc of 66 K [3] , which can be increased up to 82 K by introduction of nitrate groups [4].

We report here our results on the synthesis and the characterization of new superconducting oxycarbonates containing mercury or thallium.

2. STRUCTURAL MECHANISMS

Five layered oxycarbonates have been synthesised whose composition and characterization are listed elsewhere [5]

On a structural point of view, they can be divided into two families which are closely connected. For the first one, which corresponds to the general formula Al_xMxSr4Cu2(CO3) O7-~ with A = Hg or T1 and M = Bi or Pb, the symmetry is tetragonal (space group P4/mmm) with a = ap, where ap is the lattice parameter of the cubic perovskite, and c = 16.5 A [6-9]. The structure is built up from the intergrowth of double rock salt layers [Al-xMxOl_8(SrO)2]** with two singles perovskite layers [SrCuO3]** linked through single carbonate layer (Fig. la). They can be described as the intergrowth of "1201"-type slices [AI_ xMxSr2CuO5_3]** with slices of the strontium oxycarbonate [Sr2CuO2(CO3)],~.

The second family is represented by the series T1Sr4_xBaxCu2(CO3)O7-~ (1 < x < 2) [10].

0 • 0 • 0 • 0

a

Sr2CuO2CO 3

1201

o

• iI

P II

D • I

I 0 II,

| • II

b

Figure 1. Schematic representation of the structure of the oxycarbonates Al_xMxSr4Cu2(CO3) 07 (a) and TISr4_xBaxCu2(CO3)O 7 (b). In the latter drawing, the "TI(Ba,Sr)4Cu2(CO3)O7" blocks are illustrated by rectangles.

0921-4534/94/S07.00 © 1994 - Elsevier Scicnce B.V. All rights reserved. SSDI 0921-4534(94)00830-2

5 3 8 U Miche l et al. ,l~ln'sica ( ' 235 240 ~1994~ 55 ~ 55,~

The electtron diffraction study gives evidence of an orthorhombic cell with a = ap, b = 8ap, c -~ 16.7 A and a A-type space group. The structure of these compounds is closely related to the previous one and can be obtained by a shearing mechanism along ~. It consists of blocks of "TISr4Cu207CO3", four

CuO6 octahedra wide, which are translated of - 2

with regard to the adjacent ones. If the [CuO2] layers and [SrO] layers remain unchanged and form infinite layers parallel to (001), [TIO]~ ribbons are

limited to four TI atoms along b and are connected to ribbons of four carbonate groups (Fig. lb). The difference between T1-O and C-O apical distances, 2.0 A and 1.3 A respectively, leads to a waving of the [CuO2],~ and [(Ba,Sr)O]~ layers.

0

T(K) i i i

t 1 Cjq

- 0 . 8

5O i I

a

J

3. SUPERCONDUCTING PROPERTIES

Figure 2. Real part of the ac magnetic susceptibility versus temperature for TI0.5Pb0.5Sr4Cu2(CO3)O7_ 6 before (a) and after (b) annealing.

The behaviour of these materials depends on the nature of the M cation associated with mercury or thallium. For M = Bi, Tc are rather low : 17 K and 55 K for as-synthesized materials with A= Hg and TI respectively, and does not change dramatically whatever the thermal txeatment is. For M = Pb and for the pure thallium oxycarbonate, the behaviour is completely different. As-synthesized, the lead substituted materials can be superconducting with

Tc close to 60 K (TI/Pb sample) or not superconducting (Hg/Pb sample), but in both case, Tc can be increased up to 70 K with superconducting volume fractions larger than 50 % after annealing in a reducing atmosphere at 270°C (Fig. 2). Tc can also be influenced by the oxygen nominal composi t ion . For the T1Sr4_xBax Cu2(CO3)O7-~i series, the best superconducting properties are obtained for x = 2. After optimization by annealing in reducing atmosphere a Tc of 62 K can be achieved. Then, except for Bi doped materials, it appears that the as-synthesized compounds are always overdoped.

4. CONCLUSION

Layered oxycarbonates have been isolated and studied ; the structure of which can be described from the intergrowth infinite or finite blocks of two parent structures : "1201" and Sr2CuO2CO3. It is remarkable that these materials are superconductors whereas the parent compounds are not superconductors or superconductors with lower Tc.

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