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-Na0.5Ln0.5TiO3) (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 (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 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.
Acknowledgments
This work was supported by the National Science Foundation, Division of Earth Sciences with contributions from the NSF-wide SUSchem initiative (Grant EAR-1321410).
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