THERMODYNAMIC STABILITY OF CUPRATE PEROVSKITE SUPERCONDUCTORS

The oxygen-partial-pressure versus temperature phase behaviour of a variety of cuprate perovskites, including high-T(c) superconductors has been studied. For the n = 1,2,3 and infinity Bi-compounds and the n = 2 T l-compound the temperature above which the perovskite structure becomes unstable lies on the same boundary of thermodynamic stability. This boundary coincides with that separating cupric and cuprous oxide implying that stability of the compound is determined by the stability of the copper oxide (CuO2) planes. The variety of other cuprate perovskites which remain stable to well beyond this boundary appear to be those in which the CuO2 planes are subject to substantial internal strain whether compressive or tensile. For these, stability is probably determined by the non-copper-oxide substructures.