High-Temperature Heat Capacity of Pb 9 R(GeO 4 ) 3 (VO 4 ) 3 (R = La, Pr, Nd, Sm) Apatites

The apatite-like Pb9Pr(GeO4)3(VO4)3 and Pb9Sm(GeO4)3(VO4)3 compounds were prepared by solid-phase synthesis using oxides as starting chemicals: PbO, Pr2O3 (Sm2O3), GeO2, and V2O5. The successive annealing was carried out at 773–1073 K in the air. The effect of rare-earth elements on the structure of the Pb9R(GeO4)3(VO4)3 (R = La, Pr, Nd, Sm) apatites and basic thermodynamic functions was investigated. The temperature dependence (350–1050 K) of the heat capacity of the Pr(Sm)-containing apatites has been determined by differential scanning calorimetry. It has been established that the Cp = f(T) curve for the Pb9Pr(GeO4)3(VO4)3 compound has an extremum associated with a polymorphic transformation in the region of 978 K.