Synthése et thermochimie de phosphates au cadmium: Partie II: Enthalpie standard de formation de fluorapatites calco-cadmiées

Abstract

Cadmium–Calcium fluorapatite solid solutions with the general formula Ca 10− x Cd x (PO 4) 6F 2 were prepared by a double decomposition method. The samples were characterized by X-ray diffraction, infrared spectroscopy and chemical analysis. The limit product corresponds to x=6. Using an isoperibol calorimeter, enthalpies of solution of these products in 9% weight nitric acid were measured. A thermochemical cycle is proposed and complementary processes are carried out in order to get the standard enthalpies of formation of these apatites. The variation of the enthalpy of solution versus X=Cd/(Ca+Cd) ratio, shows a minimum at about X=0,4. This is probably related to the existence, in the hexagonal apatite structure, of two types of cationic sites.