Isomorphism and phase diagram of Pb5(PO4)3F–Pb5(PO4)3Cl system

Compounds of composition Pb5(PO4)3FxCl1−x (0 ≤ x ≤ 1), which are synthetic analogues of minerals pyromorphite, flourapatite, and endlichite, were synthesized for the first time by high-temperature solid phase reactions. X-ray diffraction and IR spectroscopy were used to determine the structure of the compounds and revealed complete miscibility in the solid phase of the Pb5(PO4)3F–Pb5(PO4)3Cl binary system. Adiabatic reaction calorimetry was used to measure standard enthalpies of mixing and formation and show that the regular solutions model is applicable to the Pb5(PO4)3F–Pb5(PO4)3Cl system. Differential thermal analysis in tandem with high-temperature X-ray diffraction were used to study the phase diagram and characterize phase transitions.