Thermodynamic model and structure of CaO–P2O5 glasses

The distribution of Q-units of CaO–P2O5 glasses was described by the thermodynamic model of Shakhmatkin and Vedishcheva. The glass was considered as the ideal solution of CaO, P2O5, CaP2O6, Ca2P2O7, and Ca3P2O8. In the first step, molar Gibbs energies of considered species were taken from the FACT thermodynamic database. The obtained result was compared with 31P solid-state NMR study of Roiland. It was shown that the calculated values were in fairly good agreement with the experimental values. After that, the nonlinear regression treatment was used for optimization of molar Gibbs energies by minimizing the sum of squares of deviations between experimental and calculated Q-distribution. In such a manner, the non-ideality of the system was reflected. In the studied case, no significant improvement of obtained results was achieved by this procedure—thus, the ideal solution assumption included in the thermodynamic model of Shakhmatkin and Vedishcheva holds very well for the studied binary glasses.