Abstract
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-Na0.5 Ln 0.5TiO3) ( Ln = La, Ce, Pr, Nd). In this study, perovskite solid solutions Ca1−2xNaxLaxTiO3 (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 ![Formula][1] and from the elements ![Formula][2] have been determined using high-temperature oxide melt solution calorimetry in molten 3Na2O·4MoO3 at 973 K. Enthalpy of formation from oxides ![Formula][3] 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 Ca1−2xNaxLaxTiO3 (0 ≤ x ≤ 0.50) along the CaTiO3-Na0.5La0.5TiO3 join provides insights into the natural occurrence of loparite minerals (La,Na,Sr,Ca)(Ti,Nb,Ta,Fe3+)O3. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif [3]: /embed/mml-math-3.gif
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