Phase equilibria in the system SrO-CaO-CuO have been studied under high oxygen pressures. The recently discovered non-superconducting binary phase,
Sk7413, Cu02, which is stable only under high oxygen pressure, forms a complete range of solid solutions with the structurally related phase, Ca,.,,,,,CuO,. These materials have complicated superstructures and may form a family of Vernier phases, such as wlas found in Sr,.,4,,,Cu02.
Stabilities and oxygen stoichiometries of tetragonal Srz_zXCazXCuOj+~ (0.0 I x 2 0.1) and Sr,4_,4xCa14xCu2qOjg+s (04 I x 5 O-S) solid solutions under high oxygen pressures are also discussed, 1 Introduction
Discovery of high temperature superconductivity in Sr-Cu-0 samples prepared under pressure has renewed interest in the study of Sr-Cu-0 and related phase diagrams, under both high isostatic pressure and high oxygen pressures.'-" In air, at one bar, there are three phases in the SrO-CuO system with compositions Sr,CuO,, SrCuOz and Sr14Cu24%+s, all of which exhibit extensive Ca e Sr substitution.&" Hiroi et al3 and oth-ers4,'2,'3 reported superconductivity in Srn+'Cu,02n+l+G (n = 1, 2, 3...-), prepared under high isostatic pressures (> 5 GPa), using KC104 as an oxidiser. Orthorhombic, non-superconducting Sr,CuO,, which forms in air at ambient pressure, transforms to tetragonal superconducting Sr2Cu03+, (6 = 0. l), i.e. n = 1, when heated at 800-900ยฐC under high pressure.3,4,'2.'3 This phase has a highly oxygendeficient K,NiF, structure. A similar product prepared at 370ยฐC in flowing oxygen using a copper hydroxometallate precursor, Sr,Cu(OH), had an oxygen excess of 6 = 0.29 and was non-superconducting.14 The orthorhombic to tetragonal phase