Enthalpie De Formation Et De Melange De Phosphoapatites Calco-Cadmiees Chlorees

Calcium–cadmium chlorapatites solid solutions with the general formula Ca_10–xCd_x(PO₄)₆Cl₂,1≤x≤10, were prepared by solid state reaction and characterized by X-ray diffraction, infrared spectroscopy and chemical analysis. Using an isoperibol calorimeter, their enthalpies of solution in 9 mass% nitric acid were measured. In order to determine the enthalpies of formation and enthalpies of mixing, thermochemical cycles were proposed and complementary experiences were performed. The results obtained show a decrease of the enthalpy of formation with the amount of cadmium introduced in the lattice. The variation of mixing enthalpy vs. x=Cd/(Cd+Ca) shows a maximum at about x=0,4. This could be explained by the existence of two cationic sites in the phosphoapatite structure. This revised version was published online in August 2006 with corrections to the Cover Date.