A thermochemical study of La_2-xA_xCuO_4-y (A=Ba, Sr, Ca, Pb)

The compounds La_2-xA_xCuO_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-xSr_xCuO_4-y, the enthalpy of oxidation (formation of formal Cu³⁺) is -129±20 kJ/(mole O₂), 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-xBa_xCuO_4-y, the enthalpy of oxidation is -220±50 kJ/(mole O₂), while the enthalpy of the charge balanced substitution is 0±10 as in the strontium doped system. Apparent phase relationships in the La_2-xA_xCuO_4-y (A=Ba, Sr, Ca, Pb) pseudobinary system are interpreted in terms of these thermodynamic properties.