Heat capacity of with magnetoplumbite structure has been measured by relaxation, adiabatic and differential scanning calorimetries in the temperature range 2–1848 K. Temperature dependences of entropy, enthalpy change and derived Gibbs energy were calculated based on the smoothed heat capacity values. Enthalpy of formation from binary oxides and standard enthalpy of formation at 298 K were estimated from enthalpy of dissolution in lead borate melt measured using the drop-solution calorimetry at 1073. To determine the probability of the decomposition of into binary oxides, the temperature dependence of the Gibbs energy of the solid-phase reaction in the temperature range of 298–1800 K was estimated.
P.G. Gagarin (Wed,) studied this question.