Enthalpies of formation of the solid solutions of ZrxY0.5-x/2Ta0.5-x/2O2 (0 ≤ x ≤ 0.2 and 0.65 ≤ x ≤ 1)

Enthalpies of formation and heat capacities are key parameters for thermodynamic assessments using the CALPHAD method and were determined in this work in the ZrO2-Y0.5Ta0.5O2 quasibinary system. ZrxY0.5-x/2Ta0.5-x/2O2 samples with compositions in the monoclinic and tetragonal ZrO2-based solid solutions (0.65 <= x <= 1) as well as the two monoclinic (M' and M) polymorphs of the Y0.5Ta0.5O2-based solid solutions (0 <= x <= 0.2) were investigated. The enthalpies of the two monoclinic polymorphs M' and M are essentially the same within experimental uncertainty and have significant energetic stability (enthalpies of formation near -45 kJ/mol) relative to their component oxides. The monoclinic and tetragonal zirconia-rich solid solutions have little energetic stability with respect to ZrO2, Ta2O5, and Y2O3 and presumably owe their existence to the configurational entropy. The heat capacities of M' and M phase are similar and can be estimated from their constituent oxides using the Neumann-Kopp rule.