Tight-binding calculation of formation energies in multicomponent oxides: Application to the MgO-CaO phase diagram

In this work we illustrate how the tight-binding formalism can be used to accurately compute formation energies in multicomponent oxides. The large and complex unit cells of these systems make the application of precise ab initio local-density approximation (LDA) techniques very computer-time demanding. Consequently, simpler but less accurate methods, such as potential models, are usually used but at the expense of obtaining just qualitatively correct results. In addition, many ad hoc assumptions have to be taken in potential models to deal with charge transfer and many-body effects. The tight-binding method can naturally incorporate these effects in its formalism and still retain a low computational cost. Furthermore, we will show that it can accurately describe the energetics of oxides once its parameters are fitted to precise LDA calculations. These considerations are important when computing temperature-composition phase diagrams since formation energies of many ordered structures are the crucial input to the calculation. We used the CaO-MgO system as a prototype system to illustrate the effectiveness of the tight-binding method. We show that by fitting its parameters to ab initio plane-wave pseudopotential calculations for a few simple ordered structures the formation energy of more complex structures can be easily predicted. We also compute the solubility limits of the CaO-MgO phase diagram. \textcopyright{} 1996 The American Physical Society.