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,PO_7/2⁻²,PO_5/2)_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.