Critical evaluation and thermodynamic assessment of the R2O–P2O5 (R = Li, Na and K) systems

The R2O–P2O5 (R = Li, Na and K) systems are thermodynamically optimized based on the evaluated phase equilibria and thermodynamic data by the CALPHAD method. Liquid phase is described by the Modified Quasichemical Model, which takes short-range ordering in liquid solution into account. All intermediate phases RPO3, R5P3O10, R4P2O7 and R3PO4 are treated as stoichiometric compounds and the corresponding polymorphic transitions are considered. A set of self-consistent model parameters for describing the Gibbs free energy of each phase are obtained. The experimental phase equilibria, enthalpy of formation, entropy, heat capacity and activity are reproduced well within experimental error limits. The calculated liquidus around compounds become flatter with the increase of P2O5 content, suggesting that the degree of stability of phosphate increases as its composition approaches R3PO4. The calculated enthalpies of formation for intermediate compounds become more negative and entropies become more positive with the increase of atomic numbers of alkali metals.