Studies of vibrational surface modes in ionic crystals. I. Detailed shell-model studies for the unrelaxed (001) face of seven crystals having the rocksalt structure
- 15 January 1977
- journal article
- research article
- Published by American Physical Society (APS) in Physical Review B
- Vol. 15 (2), 1167-1186
- https://doi.org/10.1103/physrevb.15.1167
Abstract
We present a comprehensive, unified account of the lattice dynamics of unrelaxed (001) faces of seven crystals having the rocksalt structure, three of which have an absolute gap between the acoustical and optical bulk bands (RbF, RbCl, and NaI) and four of which have overlap between these bulk bands (NaF, NaCl, LiF, and MgO). The results are obtained from calculations of the normal modes of a thin crystal film in which the ionic interactions are described by the rigid-shell model. An outline of the formulation of the shell model for the film is given, and the rapid convergence of surface-phonon frequencies with increasing film thickness is demonstrated. Surface- and pseudosurface-phonon dispersion curves and bulk bands are displayed in the form of the dispersion curves of a 15-layer film over the two-dimensional surface Brillouin zone. The surface-excess phonon density of states is given, and its peaks and valleys are correlated with features in the film dispersion curves. The domains of existence and the vibrational character of the more prominent surface-phonon bands are discussed. We establish the relation between the surface modes of the lattice models and the macroscopic surface waves of the dielectric and elastic continuum theories (Fuchs-Kliewer and Rayleigh waves, respectively). Comparison with experiments is made; the agreement with low-energy atom scattering from single-crystal surfaces of LiF and with inelastic neutron scattering from microcrystallite samples of MgO is particularly notable, although the latter comparison also displays some significant discrepancies.
Keywords
This publication has 53 references indexed in Scilit:
- Observation of Vibrational Surface Modes in the Acousto-optical Bulk Gap of TiNPhysical Review Letters, 1975
- Infrared Absorption in Small Ionic CrystalsPhysica Status Solidi (b), 1972
- Establishment of an Ordered Array of Physisorbed Ethanol Molecules on Lithium Fluoride as Evidenced by the Diffraction of Helium AtomsThe Journal of Chemical Physics, 1972
- Scattering of Helium Atoms by Phonons in the (001) Surface of LiF at 150°KThe Journal of Chemical Physics, 1971
- Vibrational surface thermodynamic functions of magnesium oxideSurface Science, 1971
- Optical Surface Phonons in Zinc Oxide Detected by Slow-Electron SpectroscopyPhysical Review Letters, 1970
- Interaction of Finite NaCl Crystals with Infrared RadiationPhysical Review B, 1970
- Infrared Absorption in Small KCl CrystalsPhysical Review B, 1969
- Anregung von Kristallgitterschwingungen durch ElektronenstrahlenThe European Physical Journal A, 1968
- Search for Surface Modes of Lattice Vibrations in Magnesium OxidePhysical Review Letters, 1968