Thermochemistry of rare-earth orthophosphates

The enthalpies of formation for the compounds (RE 3+ )PO 4 , (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)PO 4 , RE oxides, and P 2 O 5 . Enthalpies of formation for the (RE)PO 4 compounds with respect to the oxides at 298 K become more negative with increasing RE 3+ ionic radius; i.e., in going from ScPO 4 (−209.8 ± 1.0 kJ/mol), to LuPO 4 (−263.9 ± 1.9 kJ/mol), to LaPO 4 (−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.

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