Isomorphism and phase diagram of the Pb5(PO4)3Cl-Pb5(VO4)3Cl system

Compounds of composition Pb5(PxV1−xO4)3Cl (0 ≤ x ≤ 1), which are synthetic analogues of minerals pyromorphite, vanadinite, 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)3Cl-Pb5(VO4)3Cl binary system. Adiabatic reaction calorimetry was used to determine standard enthalpies of mixing and formation and showed that the regular solutions model is applicable to the Pb5(PO4)3Cl-Pb5(VO4)3Cl system. Differential thermal analysis in tandem with high-temperature X-ray diffraction was used to study the phase diagram and characterize phase transitions.