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 1:1:2 compos ition was studied at high pressures with the goal of stabilizing new p erovskite cuprates with two-dimensional ordering of La, Ca cations and oxygen vacancies. Several phases, including the perovskites La4Ca4Cu8 O20, La4Ca4Cu88O18 and the Ruddlesden-Popper (RP) phase La2Ca2Cu3O8, c ontaining three copper oxide layers (n = 3), were revealed for the fir st time. The results are very sensitive to experimental conditions, an d a variety of reaction channels are observed at the 1:1:2 composition depending on the choice of total pressure, p(O)2, temperature, and an nealing conditions. The perovskite-related phases at this composition exhibited A-site cation disorder and three-dimensional ordering of oxy gen vacancies. The Ruddlesden-Popper phase required the substitution o f 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 carr ier concentration at which superconductivity might be observed. (C) 19 94 Academic Press, Inc.