Energy of <100> Coincidence Twist Boundaries in Transition Metal Oxides

摘要

The energies of coincidence twist boundaries formed by rotation about a <100> axis in stoichiometric transition metal oxides crystallizing in the NaCl structure (such as NiO, MnO, and CoO) have been calculated using the ionic-type potentials for these materials. From a relaxation calculation it is found that the energies of the most prominent coincidence twist boundaries at 36.87, 22.62, 28.07, and 16.26 exp 0 (which correspond to inverse coincidence-site densities of sigma = 5, 13, 17, and 25, respectively) lie above the energy of the free (100) surface. The reason arises from the repulsive Coulomb interactions between ions in configurations close to anti-coincidence (anion on anion or cation on cation) which are capable of overcoming the attractive Coulomb interactions between the ions in coincidence configurations (cation on anion and vice versa). It is concluded that in contrast to metals the structures of <100> twist boundaries in oxides and alkali halides may not simply be derived from the coincidence lattice geometry. Entropy terms are not considered. (ERA citation 05:006004)