Solid state chemistry of new perovskite and ruddlesden-popper phases in the la₂o₃-cao-cuo system at high pressures

The crystal chemistry of the system LaO1.5-CaO-CuO at the 1:1 composition was studied at high pressures with the goal of stabilizing new perovskite cuprates with two-dimensional ordering of La, Ca cations and oxygen vacancies. Several phases, including the perovskites La4Ca4Cu8O20, La4Ca4Cu88O18 and the Ruddlesden-Popper (RP) phase La2Ca2Cu3O8, containing three copper oxide layers (n = 3), were revealed for the first time. The results are very sensitive to experimental conditions, and a variety of reaction channels are observed at the 1:1 composition depending on the choice of total pressure, Po2, temperature, and annealing conditions. The perovskite-related phases at this composition exhibited A-site cation disorder and three-dimensional ordering of oxygen vacancies. The Ruddlesden-Popper phase required the substitution of Sr on the A-sites to be metastably retained at room temperature. Due to its thermal instability, the RP phase could not be doped to a carrier concentration at which superconductivity might be observed. © 1994 Academic Press, Inc.