Rare-earth perovskites along the CaTiO₃-Na_0.5La_0.5TiO₃ join: Phase transitions, formation enthalpies, and implications for loparite minerals

The mineral loparite is the principal ore of the light rare earth elements (LREE) with solid solution compositions in the quaternary system perovskite-lueshite-tausonite-loparite (CaTiO₃-NaNbO₃-SrTiO₃-Na_0.5Ln_0.5TiO₃) (Ln = La, Ce, Pr, Nd). In this study, perovskite solid solutions Ca_1-2xNa_xLa_xTiO₃ (0 ≤ x ≤ 0.50) along the perovskite-loparite join were synthesized using solid-state sintering methods. XRD analysis indicates that as the Na+La content increases, the structure changes from orthorhombic to tetragonal. The enthalpies of formation at 298 K from the constituent oxides (ΔH°_{f, ox}) and from the elements (ΔH°_f,el) have been determined using high-temperature oxide melt solution calorimetry in molten 3Na₂O.4MoO₃ at 973 K. Enthalpy of formation from oxides ΔH°_f,ox becomes more exothermic with increasing Na+La content, suggesting a stabilization effect of the substitution Ca²⁺ → 0.5Na⁺ + 0.5La³⁺. The observed trend of increasing thermodynamic stability with decreasing structural distortion is in agreement with that seen in many other ABO₃ perovskites. The thermodynamic stability of perovskite solid solutions Ca_1-2xNa_xLa_xTiO₃ (0 ≤ x ≤ 0.50) along the CaTiO₃-Na_0.5La_0.5TiO₃ join provides insights into the natural occurrence of loparite minerals (La, Na, Sr, Ca)(Ti, Nb, Ta, Fe³⁺)O₃.