Enthalpies of formation of lanthanide oxyapatite phases

A family of lanthanide silicates adopts an oxyapatitelike structure with st ructural formula Ln(9.33)square (0.67)(SiO4)(6)O-2 (Ln = La, Sm, Nd, Gd, sq uare = vacancy). The enthalpies of solution, DeltaH(S), 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 d ata were used to complete thermodynamic cycles to calculate enthalpies of f ormation from the oxides, DeltaH(f-oxides)(0) (kJ/mol): La(9.33)square (0.6 7)(SiO4)(6)O-2 = -776.3 +/- 17.9, Nd(9.33)square (0.67)(SiO4)(6)O-2 = -760. 4 +/- 31.9, Sm(9.33)square (0.67)(SiO4)(6)O-2 = -590.3 +/- 18.6, and Gd(9.3 3)square (0.67)(SiO4)(6)O-2 = -446.9 +/- 21.9. Reference data were used to calculate the standard enthalpies of formation from the elements, DeltaH(f) (0) (kJ/mol): La(9.33)square (0.67)(SiO4)(6)O-2 = 14611.0 +/- 19.4, Nd(9.3) 3 square (0.67)(SiO4)(6)O-2 = -14661.5 +/- 32.2, Sn(9.33)square (0.67)(SiO4 )(6)O-2 = -14561.7 +/- 20.8, and Gd(9.33)square (0.67)(SiO4)(6)O-2 = -14402 .7 +/- 28.2. The formation enthalpies become more endothermic as the ionic radius of the lanthanide ion decreases.