Experimental study and thermodynamic assessment of the MgO-Al2O3-P2O5 system

The phase relationships of the MgO-Al₂O₃-P₂O₅ system at 1473 K was determined using the quenching method. The ternary compound MgAl(PO₄)O was considered in this study, which had not been addressed in previous reports. The phase diagram of the MgO-Al₂O₃-P₂O₅ system was thermodynamically assessed using the CALculation of PHAse Diagram (CALPHAD) method with Thermo-Calc software, incorporating both the experimental data from this study and existing literature data. An ionic two-sublattice model, (Mg⁺², Al⁺³)_P(O⁻²,AlO₂⁻¹,PO₄⁻³,PO_7/2⁻²,PO₃⁻¹,PO_5/2)_Q was used to described the liquid phase of the MgO-Al₂O₃-P₂O₅ system. The excess Gibbs energy was formulated by the Redlich-Kister expression. All solid intermediate compounds containing phosphorus were treated as stoichiometric compounds. The Gibbs energy of intermediate compounds could be modeled following Neumann-Kopp's rule. A set of self-consistent thermodynamic parameters for the MgO-Al₂O₃-P₂O₅ system was obtained and the calculated phase diagrams were in good agreement with the experimental data. This work contributes to the construction of the database for high-order systems, thereby facilitating the synthesis of phosphorus-containing materials.