A thermochemical study of La 2−xA xCuO 4−y ( A=Ba, Sr, Ca, Pb)

Abstract

The compounds La 2−x A x CuO 4−y ( A =Ba, Sr, Ca, Pb) were synthesized in their easily accessible single phase regions. High temperature solution calorimetry was performed to determine the enthalpies of formation at 704°C from oxide components. For small values of x (x <-0.1), the enthalpy of formation for Ba, Sr, and Ca substituted materials becomes progressively more exothermic with x . In contrast, Pb substituted materials show no change in the enthalpy of formation. For higher values of x (x >0.1), the enthalpy of formation of the Ba, Sr, and Ca substituted materials differs significantly. These enthalpy trends are interpreted as a balance of effects due to oxidation, oxygen vacancy formation, and oxygen vacancy ordering. The former depends strongly on the basicity of the divalent cation. For La 2−x Sr x CuO 4−y , the enthalpy of oxidation (formation of formal Cu 3+ ) is −129±20 kJ/(mole O 2 ), the enthalpy of the charge balanced substitution resulting in oxygen vacancy formation is 0±10, and the enthalpy of oxygen vacancy ordering is −40±10 kJ/(mole O 2− vacancy). For La 2−x Ba x CuO 4−y , the enthalpy of oxidation is −220±50 kJ/(mole O 2 ), while the enthalpy of the charge balanced substitution is 0±10 as in the strontium doped system. Apparent phase relationships in the La 2−x A x CuO 4−y ( A =Ba, Sr, Ca, Pb) pseudobinary system are interpreted in terms of these thermodynamic properties.