Thermodynamic functions of Ba10(PO4)6Cl2, Sr10(PO4)6Cl2 and Ca10(PO4)6Cl2

Alkaline earth chloroapatites, M10(PO4)6Cl2 (M = Ba, Sr, Ca) were prepared by solid state reaction route and characterized by XRD. The lattice constants of Ba10(PO4)6Cl2, Sr10(PO4)6Cl2, Ca10(PO4)6Cl2 were determined to be a = 10.27(1) A, c = 7.66(1) A; a = 9.87(2) A, c = 7.18(1) A and a = 9.52(1) A, c = 6.85(1) A respectively. The enthalpy increments were measured by inverse drop calorimetric method in the temperature range 523–1423 K using a high temperature calorimeter. Thermodynamic functions such as heat capacity, entropy and Gibbs energy functions in the temperature range 298–1500 K, were computed from the measured enthalpy increments.