Thermochemistry of Rare Earth Perovskites

Abstract

The rare earth (RE) mineral loparite with the chemical composition (RE, Na, Sr, Ca)(Ti, Nb, Ta, Fe⁺³)O₃ is the principal ore of the light rare earth elements (LREE) as well as niobium and tantalum. The enthalpies of formation of RE_0.67−xNa_3xTiO₃ (RE = La, Ce) and Ca_1−2xNa_xLa_xTiO₃ from oxides and elements of lanthanum and cerium perovskites and their solid solutions have been obtained using high temperature oxide melt solution calorimetry. RE_067−xNa_3xTiO₃ (RE = La, Ce) perovskites become more stable relative to oxide components as sodium content increases. Na_0.5Ce_0.5TiO₃ and Na_0.5La_0.5TiO₃ can be considered stable endmembers in natural loparite minerals. For perovskite solid solutions Ca_1−2xNa_xLa_xTiO₃, the enthalpies of formation from the constituent oxides $\Delta {\rm{H}}_{{\rm{f,ox}}}^ \circ$ become more exothermic with increasing Na+La content, suggesting a stabilizing effect of the substitution 2Ca²⁺ → Na⁺ + La³⁺ on the perovskite structure. The trend of increasing thermodynamic stability with decreasing structural distortion is similar to that seen in many other ABO₃ perovskites.