Enthalpies of formation of lanthanide oxyapatite phases

A family of lanthanide silicates adopts an oxyapatitelike structure with structural formula Ln_{9.33 0.67}(SiO₄)₆O₂ (Ln = La, Sm, Nd, Gd, = vacancy). The enthalpies of solution, ΔH_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 data were used to complete thermodynamic cycles to calculate enthalpies of formation from the oxides, ΔH0 f-oxides (kJ/mol): La_{9.33 0.67}(SiO₄)₆O₂ = 776.3 ± 17.9, Nd_{9.33 0.67}(SiO₄)₆O₂ = 760.4 ± 31.9, Sm_{9.33 0.67}(SiO₄)₆O2 = 590.3 ± 18.6, and Gd_{9.33 0.67}(SiO₄)₆O₂ = 446.9 ± 21.9. Reference data were used to calculate the standard enthalpies of formation from the elements, ΔH0 _f (kJ/mol): La_{9.33 0.67}(SiO₄)₆O₂ = 14611.0 ± 19.4, Nd_{9.33 0.67}(SiO₄)₆O₂ = 14661.5 ± 32.2, Sm_{9.33 0.67}(SiO₄)₆O₂ = -14561.7 ±; 20.8, and Gd_{9.33 0.67}(SiO₄)₆O₂ = -14402.7 ± 28.2. The formation enthalpies become more endothermic as the ionic radius of the lanthanide ion decreases.