Standard enthalpy, entropy and Gibbs free energy of formation of «A» type carbonate phosphocalcium hydroxyapatites

«A» type carbonate phosphocalcium hydroxyapatites having the general formula Ca10(PO4)6(OH)(2-2x)(CO3)x with 0 ⩽ x ⩽ 1, were prepared by solid gas reaction in the temperature range of 700-1000 °C. The obtained materials were characterized by X-ray diffraction and infrared spectroscopy. The carbonate content has been determined by C-H-N analysis. The heat of solution of these products was measured at T = 298 K in 9 wt% nitric acid solution using an isoperibol calorimeter. A thermochemical cycle was proposed and complementary experiences were performed in order to access to the standard enthalpies of formation of these phosphates. The results were compared to those previously obtained on apatites containing strontium and barium and show a decrease with the carbonate amount introduced in the lattice. This quantity becomes more negative as the ratio of substitution increases. Estimation of the entropy of formation allowed the determination of standard Gibbs free energy of formation of these compounds. The study showed that the substitution of hydroxyl by carbonate ions contributes to the stabilisation of the apatite structure.