Energetics of lanthanum silicate apatite: Influence of interstitial oxygen and cation vacancy concentrations in La_{9.33 + x}(SiO₄) ₆O_{2 + 3x / 2} and La_{10 - X}Sr _x(SiO₄)₆O_{3 - 0.5x}

Lanthanum silicate apatite materials, which are good ionic conductors, were prepared and characterized in two series, LSO: La_{9.33 + x}(SiO ₄)₆O_{2 + 3x / 2} (x = 0, 0.33 and 0.67) and LSSO: La_{10 - x}Sr_x(SiO₄)₆O _{3 - 0.5x} (x = 1.00 and 2.00). Enthalpies of formation from their binary oxides at 298 K were determined by high temperature oxide melt solution calorimetry. The energetics of these materials is discussed in terms of the effects of cation vacancies and interstitial oxygens. The formation of LSO apatites becomes more exothermic as the number of cation vacancies decreases (oxygen excess increases). Cation vacancy content is the dominant factor in determining the energetics of La_{9.33 + x}(SiO₄) ₆O_{2 + 3x / 2}. The endothermic enthalpy of defect formation is 133.8 ± 17.5 kJ per mole of interstitial oxide ions, and 272.2 ± 21.6 kJ per mole of cation vacancies. Thus, La₈Sr ₂(SiO₄)₆O₂, having neither vacancies nor interstitial oxide ions, is the most stable compound in these two series.