Lattice dynamics of a commensurate interface between two ionic crystals

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Abstract
The theory of lattice vibrations localized at the interface of two ionic crystals with commensurate surface periodicities is developed using a Green's-function approach. The use of a perturbation matrix averaged over the interface supercell enables one to give an approximate description of interface modes in terms of plane waves. The wave vectors vary over an extended-surface Brillouin zone defined as the supercell of the two reciprocal surface lattices. The dispersion curves and the projected phonon densities are calculated in the framework of the breathing-shell model for the LiF(001)/KF(001) interface, where the lattice constants are approximately in the ratio 3/4. Strong interface resonant modes are found above the sagittal transverse bands of the heavier crystal. These modes, rather than Stoneley waves, appear to be the most important feature of interface dynamics. Some consequences of the interface dynamical structure on the Brillouin scattering spectra of ionic crystal intercalates are briefly discussed.