Energy and structure of (001) coincident-site twist boundaries and the free (001) surface in MgO: A theoretical study

A computer code is described which permits investigation of the energy and structure of coincident-site lattice (CSL) grain boundaries, stacking faults, and free surfaces in ionic crystals. The code uses computational techniques similar to those of Harwell's HADES code for the study of point defects in bulk ionic crystals. Two sets of short-range potentials are used to determine the energy and structure of the free (001) surface and of (001) CSL twist boundaries in MgO with values of Σ, the inverse density of CSL sites, ranging between Σ = 5 and Σ = 65. From comparison of the results obtained by means of the different potentials it is concluded that the Van der Waals attraction between oxygen ions on opposite sides of the interface is mainly responsible for the rather weak cohesion predicted for such bicrystals, whereas Coulombic interactions are found to play only a minor role. It then follows that (i) the (001) plane is not a favored plane for the formation of grain boundaries in pure oxides with NaCl structure and (ii) similarities should exist between (001) twist boundaries in ionic crystals with NaCl structure and in fcc metals. These similarities are investigated by comparing calculated boundary structures and energy-vs-misfit angle curves for MgO with recent results for aluminum, copper, silver, and gold, in which a broad spectrum of different interatomic potentials was used. It is suggested that the rather strong cohesion of the pressure-sintered MgO bicrystals of Sun and Balluffi may be due to impurities which have migrated to the boundary during sintering