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We present a force field for bulk alumina (Al2O3), which has been parametrized by fitting the energies, forces, and stresses of a large database of reference configurations to those calculated with density functional theory (DFT). We use a functional form that is simpler and computationally more efficient than some existing models of alumina parametrized by a similar technique. Nevertheless, we demonstrate an accuracy of our potential that is comparable to those existing models and to DFT. We present calculations of crystal structures and energies, elastic constants, phonon spectra, thermal expansion, and point defect formation energies.

The well-known corrosion resistance and biocompatibility of TiN depend on the structural and chemical properties of the stable oxide film that forms spontaneously on its surface after exposure to air. In the present work, we focus on the atomistic structure and stability of the TiN(100) surface in contact with an oxidizing atmosphere. The early oxidation stages of TiN(100) are investigated by means of first-principles molecular dynamics (FPMD). We observe selective oxidation of Ti atoms and formation of an ultrathin Ti oxide layer, while Ti vacancies are left behind at the metal/oxide interface. Within the formalism of ab initio thermodynamics we compute the segregation energies of vacancies and vacancy clusters at the metal/oxide interface, comparing the stability of the system obtained by FPMD simulations with ideally reconstructed models. We find that the localization of Ti vacancies in the thin oxide layer and at the TiN/oxide interface is thermodynamically stable and may account for the early removal of N atoms from the interface by segregation of N vacancies from the bulk reservoir. We suggest that superficial oxidation may proceed along two distinct possible pathways: a thermodynamically stable path along the potential energy minimum surface and a metastable, kinetically driven path that results from the high heat release during the dissociation of O(2).

A new compressible ion model for describing the energetic components of the cohesive energy of a fully ionic crystal is developed and tested using previous ab initio results for three cubic phases of MgO. This model is physically highly transparent and improves on previous compressible ion models in two ways. First, the short-range cation–anion interaction and the rearrangement energy needed to convert a free O− ion plus a free electron into an O−2 ion having a form optimal for its in-crystal environment are decomposed into the major contributions originating from the six outermost anion electrons plus smaller terms generated by the two 2s electrons. This model transfers to the B2 and B3 phases of MgO after parametrization on the ab initio data for the B1 phase even more accurately than previous compressible ion models. Second, the separate modeling of the repulsive (permutation) and attractive (penetration) components of the short-range anion–anion interactions enables the new model to describe their sub...