Energetics of lanthanum silicate apatite: Influence of interstitial oxygen and cation vacancy concentrations in La9.33 + x(SiO4)6O2 + 3x / 2 and La10 − xSrx(SiO4)6O3 − 0.5x

Lanthanum silicate apatite materials, which are good ionic conductors, were prepared and characterized in two series, LSO: La9.33 + x(SiO4)6O2 + 3x / 2 (x = 0, 0.33 and 0.67) and LSSO: La10 − xSrx(SiO4)6O3 − 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 La9.33 + x(SiO4)6O2 + 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, La8Sr2(SiO4)6O2, having neither vacancies nor interstitial oxide ions, is the most stable compound in these two series.

► Energetics of La9.33+x(SiO4)6O2+3x/2 and La10-xSrx(SiO4)6O3-0.5x are investigated.
► Stoichiometric sample La8Sr2(SiO4)6O2 is the most stable composition.
► ∆H°f, interstitial and ΔHf, cationvacancy were determined.
► Cation vacancy concentrations appear to be the dominant factor in energetics.
► Energetics in LSSO series directly correlates with conductivity.