Thermochemistry of rare-earth-metal-alkaline-earth-metal-copper oxide superconductors

Enthalpies of formation of the perovskite-related oxides La/sub 2/CuO/sub 4/, La/sub 1.85/Sr/sub 0.15/CuO/sub 4/, and YBa/sub 2/Cu/sub 3/O/sub y/(y = 6.25, 6.47, 6.69, and 6.93) have been determined at 298.15 K by solution calorimetry. Room-temperature stabilities of these compounds have been assessed in terms of the parent binary oxides and of the oxygen content. High-temperature (to 900/degree/C) thermal behavior of YBa/sub 2/Cu/sub 3/O/sub y/ has been used to determine thermodynamic properties (partial molal enthalpy, free energy, and entropy of oxygen) of this material in order to characterize the quenched state in relation to the high-temperature equilibrium state. The thermochemistry of oxygen has been used to interpret the relationship between the oxygen partial pressure and the defect chemistry in the nonstoichiometric phase of YBa/sub 2/Cu/sub 3/O/sub y/. The relevance of the defect chemistry to conductivity is discussed. 34 references, 2 tables.