Thermochemistry of rare earth perovskites Na_3xRE_0.67-xTiO₃ (RE = La, Ce)

High-temperature oxide melt solution calorimetry using sodium molybdate (3Na₂O·4MoO₃) solvent at 973 K was performed for the Na_3xRE_0.67-xTiO₃ (RE = La, Ce) perovskite series. The enthalpies of formation of lanthanum perovskites from oxides (La₂O₃, Na₂O, TiO₂), are -107.25 ± 2.56, -93.83 ± 6.06, -80.68 ± 5.93, and -33.49 ± 4.26 kJ/mol and enthalpies of formation from elements are -1614.05 ± 5.37, -1596.44 ± 7.68, -1594.03 ± 7.58, and -1577.56 ± 6.36 kJ/mol for Na_0.459La_0.522Ti_0.999O₃, Na_0.454La_0.523Ti_0.994O₃, Na_0.380La_0.567Ti_0.980O₃, and La_0.692Ti_0.979O₃, respectively. The enthalpies of formation of cerium perovskites are -99.98 ± 5.78 and -45.78 ± 3.30 kJ/mol from oxides (Ce₂O₃, Na₂O, TiO₂), and -1611.34 ± 6.90 and -1602.06 ± 2.72 kJ/mol from elements for Na_0.442Ce_0.547Ti_0.980O₃ and Ce_0.72Ti_0.96O₃. The A-site defect perovskites become more stable relative to oxide components as sodium contents increase. Na_0.5Ce_0.5TiO₃ and Na_0.5La_0.5TiO₃ could be considered as thermodynamically stable end-members in natural loparite minerals, in which these end-members are in solid solution with CaTiO₃ and other components.