Thermochemistry of rare-earth orthophosphates

The enthalpies of formation for the compounds (RE³⁺)PO₄, (where RE = Sc, Y, La–Nd, Sm–Lu) were determined by oxide-melt solution calorimetry. Calorimetric measurements were performed in a Calvet-type twin microcalorimeter in sodium molybdate (3Na₂O · 4MoO₃) and lead borate (2PbO · 2B₂O₃) solvents at 975 K. The experiments were carried out using both powdered single crystals grown by a flux technique and powders synthesized by precipitation. Formation enthalpies were derived from the drop-solution enthalpies for (RE)PO₄, RE oxides, and P₂O₅. Enthalpies of formation for the (RE)PO₄ compounds with respect to the oxides at 298 K become more negative with increasing RE³⁺ ionic radius; i.e., in going from ScPO₄ (−209.8 ± 1.0 kJ/mol), to LuPO₄ (−263.9 ± 1.9 kJ/mol), to LaPO₄ (−321.4 ± 1.6 kJ/mol). From structural considerations, a similar trend is expected for the isostructural RE vanadates and arsenates, as well as for the tetravalent actinide orthosilicates.