New layer structures in the La–Cu–O system

Abstract

Recently it was reported that samples with overall compositions given by La_1–xBa_xCuO_3–y were superconducting at the unprecedentedly high temperature of approximately 35 K¹. A following report in January 1987², indicated a superconducting temperature of 52.5 K for oxides in the same quaternary system. In this second report the phase which was supposed to be superconducting was a non-stoichiometric oxide of composition (La_0.9Ba_0.1)₂CuO_4–y. A number of other reports, for example, ref. 3, suggest that compounds exhibiting superconducting temperatures of 100 K or higher are to be expected soon. The exact nature of the superconducting phase or phases in these materials is unknown. Until crystallographically well-characterized samples are used in conductivity measurements, the possibility of constructing a theory which accurately explains the phenomenon of superconductivity in these materials will be difficult, if not impossible. Moreover, the search for new materials with enhanced properties, and the preparation of materials in a form suitable for device use will have to be approached empirically. To provide these data, a systematic investigation of phase relations in the systems of interest has been started. Here we present new data on the parent La–Cu–O system which give evidence of the existence of new layer structures of relevance to the superconducting systems described above.