Energetic effects of substitution of La-Nd and Si-Ge oxyapatite-type materials

A series of lanthanide oxyapatites, neodymium silicates (Nd _9.33-xM_3x/2(SiO₄)₆O₂; x = 0.0 and 2.0 and M = Ca, Sr, and Ba), and lanthanum germanate (La ₁₀(GeO₄)₆O₃) were prepared by a variety of heat treatments and characterized. High-temperature oxide melt solution calorimetry in molten 2PbO-B₂O₃ solvent at 1078 K was performed to determine their enthalpies of formation from constituent oxides at room temperature. The energetics of these materials is discussed in terms of the effects of doping on two crystallographic sites, the lanthanide and tetrahedral sites. The enthalpy of formation from oxides becomes less exothermic by substituting La with Nd throughout the whole series, in both doped and undoped compositions, reflecting the smaller radius and lower basicity of Nd compared with La. Cation stoichiometric lanthanum germanate apatite (La ₁₀(GeO₄)₆O₃) shows a more endothermic enthalpy of formation than the corresponding silicate, reflecting the larger radius and lower acidity of Ge than Si.