A lock-in model for the atomic structure of interphase boundaries between metals and ionic crystals

The low energy interphase boundaries between noble metals (Au, Cu) and various ionic crystals (LiF, KCl, NaCl, MgO, A1₂O₃, mica) were determined at 550°C by means of the boundary energy induced rotation of small (~ 1 μm) spheres. The systems were chosen so that the effect of lattice mismatch (varied between 1.3 and 35.1%), the effect of lattice structure (cubic/cubic and hexagonal/cubic) and chemical effects could be studied. The results obtained suggest that boundary models based on the coincidence concept are not applicable to interphase boundaries between noble metals and ionic crystals because the low energy boundaries observed were not of the coincidence type and existing coincidence orientation relationships did not result in low energy boundaries. However, the atomic structure of the low energy interphase boundaries observed may be understood in terms of the following “lock-in model”. A low energy interphase boundary results if the close packed rows of atoms at the “surface” of the metal crystal fit into the “valleys” between close packed rows of atoms at the “surface” of the ionic crystal. This model seems to predict correctly the experimentally observed correlations between the interfacial energy and the boundary inclination, the lattice mismatch and the lattice structure of the two phases involved.