Phase equilibria study and thermodynamic assessment of CaO–MgO–P2O5 system

The phase diagram of the CaO–MgO–P2O5 system was established by both experimental investigation and thermodynamic assessment. The presence of the contentious binary compound Ca4P6O19 in the CaO–P2O5 system was confirmed. The phase relationships in the CaO–MgO–P2O5 system at temperature 1150 °C and 1200 °C were studied via high-temperature quenching experiments in conjunction with X-ray diffraction (XRD) and electron probe micro-analyzer (EPMA) techniques. The CaO–MgO–P2O5 system was critically evaluated and optimized by means of the CALPHAD (CALculation of PHAse Diagrams) methodology. The ionic two-sublattice model (Ca+2, Mg+2)P (O−2,PO3−1,PO4−3,PO7/2−2,PO5/2)Q is used to describe the liquid phase in the CaO–MgO–P2O5 system due to the ionic nature of oxide melts and the presence of ions with different charges. A set of self-consistent thermodynamic parameters were obtained, showing good agreement between the experimental data and the calculated results. This study holds significant implications for guiding the manufacturing processes of phosphate ceramics.