Enthalpies of formation of the solid solutions of Zr_xY_0.5-x/2Ta_0.5-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_.5Ta_0.5O₂ quasibinary system. Zr_xY_0.5-x/2Ta_0.5-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 Y_0.5Ta_0.5O₂-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.