A simple method for computing the formation free energies of metal oxides

This paper purposes a simple procedure for obtaining the formation free energy of metal oxides in their solid states by introducing the concept of virtual solid oxygen free energy. This scheme is meant to address the fact that standard Density functional Theory (DFT) methods tend to exhibit poor accuracy for energy differences between condensed and gas phases. The idea is to define an oxygen reference state entirely in terms of solid oxide phases, so that formation free energies can be obtained without having to compute molecular energies. The proposed reference free energy can be easily obtained from both computations and experiments, thus enabling the direct integration of DFT and experimental data. Using the molybdenum system to construct this reference, we compute the Ellingham diagram for oxides of Mo, Cu, Mg, Al and Zn, which all show good agreement with experimental values. This procedure is independent of specific total energy method used, and is applicable to formation free energy calculations of any metal oxides.