ABSTRACT
Recent years have seen great advances in our ability to predict crystal structures from first principles. However, previous algorithms have focused on the prediction of bulk crystal structures, where the global minimum is the target. Here, we present a general atomistic approach to simulate in multicomponent systems the structures and free energies of grain boundaries and heterophase interfaces with fixed stoichiometric and non-stoichiometric compositions. The approach combines a new genetic algorithm using empirical interatomic potentials to explore the configurational phase space of boundaries, and thereafter refining structures and free energies with first-principles electronic structure methods. We introduce a structural order parameter to bias the genetic algorithm search away from the global minimum (which would be bulk crystal), while not favouring any particular structure types, unless they lower the energy. We demonstrate the power and efficiency of the algorithm by considering non-stoichiometric grain boundaries in a ternary oxide, SrTiO(3).
ABSTRACT
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).
