A MODEL TO COMPUTE PHASE-DIAGRAMS IN OXIDES WITH EMPIRICAL OR FIRST-PRINCIPLES ENERGY METHODS AND APPLICATION TO THE SOLUBILITY LIMITS IN THE CAO-MGO SYSTEM

The CaO-MgO system is used as a prototype system to evaluate the accur acy of several energy and entropy approximations for predicting solid- state phase diagrams in ionic materials. Configurational disorder betw een the cations is parameterized with the cluster expansion technique. The vibrational contribution to the free energy is incorporated with a harmonic model that accounts for the dependence of the vibrational d ensity of states on the cation configuration. The CaO-MgO phase diagra m can be predicted very accurately with quantum mechanical energy meth ods, without the use of any adjustable parameters. Published empirical potential parameters for the CaO-MgO system reproduce the qualitative features of the phase diagram but significantly underestimate the sol ubility limits. Parameters that reasonably reproduce the quantum mecha nical results are presented.