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Abstract
The physical nature of (001) coincident-site lattice (CSL) twist grain boundaries in alkali-halide bicrystals has been investigated. For that purpose the energies and structures of such boundaries and of the free (001) surface are determined by means of a computer code developed in recent years. All the results for the alkali halides are found to be in complete qualitative agreement with earlier calculations for MgO and for the transition-metal oxides MnO, FeO, CoO and NiO in that
(1) the grain-boundary energy versus misfit-angle curve shows no “cusps”, and
(2) larger twist angles result in larger volume expansions at the boundary. A quantitative comparison with the metal oxides shows two interesting differences, however. First, the alkali-halide bicrystals are substantially more stable; second, their volume increase at the boundary is significantly lower. Both phenomena are investigated in some detail by analysing the effect of the different contributions to the interionic pair potentials on the interfacial properties. It is found that while both the Coulomb interactions between the ions and the Born-Mayer repulsion between nearest neighbours act to destabilize such bicrystals, the Van der Waals attraction between ions of equal type on opposite sides of the interface is solely responsible for their cohesion.
