Thermodynamic Properties of LaMnAl11O19 Hexaaluminate with a Magnetoplumbite Structure

Relaxation and adiabatic calorimetry were used to measure the isobaric heat capacity of LaMnAl11O19 hexaaluminate with a magnetoplumbite structure in the temperature range 2–330 K. Thermodynamic functions are calculated from the fitted values of the heat capacity. A study of the magnetic properties of LaMnAl11O19 has revealed that the small heat capacity anomaly observed at the lowest temperatures is not related to changes in the magnetic characteristics of the hexaaluminate. Drop-solution calorimetry was used to measure the enthalpy of dissolution, from which the enthalpy of formation of LaMnAl11O19 from oxides and elements was calculated. The calculated Gibbs energy for the decomposition of lanthanum magnesium hexaaluminate into binary oxides at high temperatures is positive, which indicates the stability of the complex oxide. The effect of replacing magnesium with manganese on the thermodynamic stability of hexaaluminate with a magnetoplumbite structure was assessed.