Thermodynamics of CoO–MgO solid solutions

Abstract

Enthalpies of mixing in Co x Mg 1 − x O solid solutions at T=298 K have been determined using high temperature drop solution calorimetry in molten sodium molybdate (3Na 2O · 4MoO 3) 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 m 0 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.