Thermochemistry of rare-earth orthophosphates

The enthalpies of formation for the compounds (RE (3+))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 (3Na(2)O . 4MoO(3)) and lead borate (2PbO . 2B(2)O(3)) 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.