Enthalpies of formation of lanthanide oxyapatite phases

A family of lanthanide silicates adopts an oxyapatitelike structure with structural formula Ln9.33h0.67(SiO4)6O2 (Ln 4 La, Sm, Nd, Gd, h 4 vacancy). The enthalpies of solution, DHS, for these materials and their corresponding binary oxides were determined by high-temperature oxide melt solution calorimetry using molten 2PbO? B2O3 at 1078 K. These data were used to complete thermodynamic cycles to calculate enthalpies of formation from the oxides, DH0f-oxides (kJ/mol): La9.33h0.67(SiO4)6O2 4 −776.3 ± 17.9, Nd9.33h0.67(SiO4)6O2 4 −760.4 ± 31.9, Sm9.33h0.67(SiO4)6O2 4 −590.3 ± 18.6, and Gd9.33h0.67(SiO4)6O2 4 −446.9 ± 21.9. Reference data were used to calculate the standard enthalpies of formation from the elements, DH0f (kJ/mol)