Energetics of cerium-zirconium substitution in the xCe_0.8y _0.2O_1.9-(1 -x)Zr_0.8y_0.2O _1.9 system

Energetics of Ce-Zr substitution in xCe_0.8Y_0.2O _1.9-(1 -x)Zr_0.8Y_0.2O_1.9 (0≤x≤1) solid solutions, potential candidates for electrolytes in solid oxide fuel cells, were investigated by high temperature oxide melt drop solution calorimetry. The enthalpies of formation relative to the solid solution end-members, Ce_0.8Y_0.2O_1.9 and Zr _0.8Y_0.2O_1.9 (ΔH_f,ss), are slightly positive. The whole substitution range can be divided into three regimes: a zirconia-dominated region (0≤A:<0.4), where ΔH _f,ss increases from 0 to 3.9 kJ/mol; a ceria-dominated region (0.6<x≤1), where ΔH_f,ss remains nearly constant at 0 to 0.7 kJ/mol; and a transitional region (0.4≤x≤0.6), where ΔH _f,ss decreases from 3.9 to 0.7 kJ/mol. Such a trend in ΔH _f,ss strongly supports a scavenging effect of Zr⁴⁺ on oxygen vacancies, i.e. oxygen vacancies prefer the nearest neighboring sites of Zr⁴⁺ as in yttriastabilized zirconia. The formation enthalpy from binary oxides, fluorite CeO₂, monoclinic ZrO₂ and C-type YO_1.5, (ΔH_f,ox) is also slightly positive, varying analogously to ΔH_f,ss.