STRUCTURAL AND PHYSICAL PROPERTIES OF ... - Science Direct

Journal

of the Less-Common

213

Metals, 151 (1989) 213 - 220

STRUCTURAL AND PHYSICAL PROPERTIES THE TETRAGONAL SUPERCONDUCTORS LaBaACu,O, _ 6 (A = Ca AND Sr) *

OF

W. T. FU Kamerlingh Onnes Laboratory, (The Netherlands)

Leiden University, P.O. Box 9506, 23OORA Leiden

H. W. ZANDBERGEN Gorlaeus Laboratories, (The Netherlands)

Leiden University, P.O. Box 9506, 2300RA Leiden

C. J. VAN DER BEEK, A. A. VERHEIJEN Kamerlingh Onnes Laboratory, (The Netherlands)

and L. J. DE JONGH

Leiden University, P.O.Bor

9506, 2300RA Leiden

V. A. M. BRABERS Department of Solid State Physics, Eindhouen (The Netherlands) (Received

November

Technical

University

of Eidhouen,

P.O. Box

513, 5600MB

8, 1988)

Summary The compounds LaBaACu,O, 6 (A = Ca and Sr) have been prepared and characterized by X-ray diffraction and electron microscopy. Their structure is similar to that of YBa,Cu,O,, except that they are tetragonal. Electron diffraction of LaCaBaCu,O,_ 6 reveals the presence of a weak superstructure having doubled, a, b and c axes. The oxygen content and superconducting transition temperature were found to vary significantly, depending on the post-annealing conditions. Oxygen- and air-annealed samples have T, values of 76 K and 60 K for A = Ca and 50 K and 33 K for A = Sr respectively. Air-quenched samples exhibit semiconductor-like behaviour in both cases. The d.c. resistivity and a.c. susceptibility vs. temperature data for LaBaSrCu,O, ii are presented. The absence of an orthorhombic unit cell for these compounds is discussed and a model for the oxygen-vacancy ordering is proposed to explain the observed superstructure in LaBaCaCu,O, A,

*Paper presented at the Symposium on High Temperature Superconductors and Applications, at the E-MRS Fall Meeting, Strasbourg, November 8 - 10, 1988. 0022.5088/89/$3.50

‘$7 Elsevier

Sequoia/Printed

Preparation

in The Netherlands

214

1. Introduction After the discovery of high T, (T, > 90 K) superconductors in the family REBa,Cu,O,_ a (RE = Y and rare earth) [l - 31, the influence of oxygen stoichiometry on the superconducting properties of these materials have been extensively studied [ 4,5]. The structure of YBa,Cu,O, _ 6 can easily accommodate oxygen deficiencies over the composition range 0 < 6 < 1, having a remarkable influence on T,. In the 6 range O.O- 0.2, the structure is orthorhombic and T, is around 90 K. For the 6 range 0.3 - 0.5, T, exhibits a plateau at about 60 K and the structure remains orthorhombic, 6 > 0.5 results in a tetragonal structure with semiconducting properties [5]. In YBa,Cu,O, _ 6 the yttrium can be totally replaced by other rare earth ions, except for cerium, praseodymium and terbium, and T, remains around 90 K [3]. Partial substitution of barium by strontium has been reported by Veal et al. [6]. Single-phase material with orthorhombic symmetry was obtained up to a substitution of 50% of barium and T, decreased linearly with increasing substitution to approximately 80 K. In the La-Ba-Cu-0 system, a solid solution La, + *Ba2 ~ xCu3O7 + 6 with 0 < x < 0.5 is generally obtained as a result of the replacement of barium by lanthanum [7]. With increasing x a decrease in T, and in the degree of orthorhombicity is observed. Single-phase LaBa,Cu,O, ~ /i with T, above 90 K (orthorhombic symmetry) was obtained by Wada et al. by sintering the material in N, flow and followed by low temperature 0, annealing [8]. Ganapathi et al. reported that orthorhombic LaBa,Cu,07 _ ci with high T, (about 77 K) is found only when 6 z -0.05 [9]. However, a slight deviation of 6 (161> 0.1) results in a tetragonal symmetry and T, drops well below 50 K. In order to investigate further the influence of atom size and electropositivity and oxygen stoichiometry on the superconducting properties, we have investigated the substitution of barium by strontium and calcium in LaBa,Cu,O, p;i and found the resulting compounds LaBaSrCu,0,p6 and LaBaCaCu,O,_ 6 to be tetragonal superconductors. Some results for LaBaCaCu,O, _ d were reported earlier [ 10 - 121. In this paper we describe the preparation and some physical properties of LaBaSrCu,O, ~ 6 and compare the structural features of LaBaACu,O, ri (A = Ca and Sr) obtained by X-ray diffraction and electron diffraction.

2. Experimental

details

Samples of LaBaSrCu,O, _ s were prepared by thoroughly mixing and grinding La,O,, BaCO,, SrCO, and CuO powder in the stoichiometric ratio and firing in air at the temperature range 950 - 980 “C for two days with intermittent regrinding. Finally the samples were pressed into pellets and treated as follows: sample 1, sintering at 950 “C under oxygen flow for several hours and annealing at 450 “C for 4 h; sample 2, the same heat treatment

215

procedure but now carried out in air; sample 3, heating for several hours at 950 “C and subsequent quenching in air. The resulting materials were characterized by X-ray diffraction and electron diffraction. The oxygen content was determined by thermogravimetric analysis (TGA) by heating the samples in diluted hydrogen (8% H, in NJ at 950 “C. The procedures of the resistance and a.c. susceptibility measurements are described in ref. 10.

3. Results and discussion X-ray powder diffraction of LaBaSrCu,O,_ ii revealed the presence of only small amounts (less than 5%) of SrCuO, as a second phase. The amount of second phase could be reduced by prolonged heat treatment. All samples prepared as described above crystallize in the tetragonal structure very similar to tetragonal YBa,Cu,O, 6, and can be indexed with space group P4/mmm. The refined cell constants at room temperature from X-ray powder diffraction are given in Table 1. The c axis of the oxygenannealed sample is slightly smaller than that of the air-quenched sample whereas the difference in the a (or b) axis is not significant. A similar result for the c axis is observed for YBa,Cu,O, ii with increasing uptake of oxygen. The oxygen content as determined by TGA is presented in Table 1. The values reported here for LaBaSrCu,O, d are well below that of YBa,Cu,O,