Defect modelling: the need for angularly dependent potentials

The reliable atomistic simulation of defects in intermetallics requires the development of angularly dependent interatomic potentials. Ab initio calculations predict, for example, that the nearest competing phases to the L10 ground state of TiAl and the B2 ground state of NiAl are B33 (CrB) and B20 (FeSi) respectively, whose structural stability is determined by directional pd bonding. This is well described by the Tight Binding model, within which approximation angularly dependent many-atom interatomic potentials have recently been derived. The nature of these so-called Bond Order Potentials (BOPs) and their application to structural prediction in elemental d-valent transition metals and sp-valent semiconductors are discussed.