Thermodynamics of CoO-MgO solid solutions

Enthalpies of mixing in Co_xMg_1-xO solid solutions at T=298 K have been determined using high temperature drop solution calorimetry in molten sodium molybdate (3Na₂O·4MoO₃) solvent at T=973 K. Slightly positive enthalpies of mixing were observed, which conform to a regular solution model with interaction parameter W=(5.1±0.3) kJ·mol^-1, which is similar to the interaction parameter calculated from previously measured activity-composition relations. Standard molar entropies S_m⁰ at T=298 K calculated from subambient C_p data for CoO, MgO are (52.8±0.1) J·K ^-1·mol^-1 and (28.1±0.1) J·K ^-1·mol^-1, respectively. Integration of the C _p,m/T of the Co_0.50Mg_0.50O mixture yields an entropy of (40.3±0.1) J·K^-1·mol^-1. This value does not include configurational entropy from the Co/Mg site disorder. The excess entropy of Co_0.50Mg_0.50O is zero within experimental error. Heat capacities were also measured between T=298 K and T=973 K. The enthalpies of mixing do not depend on temperature in this range.