G. P. Alldredge

Distribution of surface phonon branches in RbF and RbCl

Abstract Predictions based on a realistic (shell) model are presented for the surface phonon branches of (001) surfaces of RbF and RbCl, RbF being the rocksalt-structure alkali halide having the largest gap between optical and acoustical frequencies in the bulk. The gap structure within the bulk bands is clarified, and new branches of LO and TO surface modes are found. Some caution is urged in the inference of details of surface mode existence based on the surface excess distribution of frequencies.

Surface Thermodynamic Functions for NaCl

The surface entropy and surface specific heat at constant volume, and the vibrational contributions to the surface energy and surface Helmholtz free energy have been calculated as functions of temperature for the (100) surface of NaCl. To our knowledge, these are the first calculations of surface thermodynamic functions for an ionic crystal to be based on a realistic lattice‐dynamical determination of the vibrational frequencies, rather than on the use of a continuum model, Einstein model, or unrealistically simple lattice model. The main approximations in the present treatment are the harmonic approximation and the neglect of the polarizabilities of the ions (i.e., the use of a rigid‐ion model). Comparison is made between the present theoretical results and the experimental results for the excess heat capacity of finely divided NaCl. At low temperatures, the present results are in good agreement with the recent data of Barkman et al. The position of the peak in the surface specific heat is in good agreem...

Surface phonons for the (001) face of lithium flouride

Abstract Surface phonon branches for the (001) face of LiF have been calculated with a shell model a fifteen-layer film. The “Lucas” TO surface-mode frequency at q = 0 is approximately equal to that of a small unexplained dip in IR transmittance of a thin LiF film as observed by Berreman.

Thermodynamic Functions and Debye–Waller Factor for Adsorbed Particles

A discussion is given of the effect of the particle masses on the thermal properties of an adsorbate–substrate system, with detailed calculations for model systems in which the adsorbate consists of a light or heavy monolayer. These models are more nearly realistic than those used in previous treatments in that there is no disagreement with the condition of rotational invariance. We have examined the dependence of the vibrational contributions to the energy, entropy, free energy, and specific heat upon the temperature, the surface orientation, and the particle masses. The mean‐square amplitudes of vibration, which determine the Debye–Waller factor, were also studied. The calculated results for the free energy and the mean‐square amplitudes are reproduced exactly at high temperatures and to a good approximation at low temperatures by a very simple model in which, roughly speaking, it is assumed that the vibrational frequencies associated with the adsorbate are proportional to the square root of the adsorba...

Self-consistent electronic structure of the (001) surface of lithium and the initial forces of ionic relaxation

Abstract We present calculations of the initial relaxing forces on the ionic planes of the (001) surface of lithium. The calculation makes use of our self-consistent conduction electron charge density for a 13-layer film, and yields a significant inward relaxing force on the outermost plane. A comparison calculation with a Morse potential shows that the Morse potential is completely inadequate in this system.

Character of Surface and Pseudosurface Waves in a Discrete Lattice

The character of surface and pseudosurface waves in a discrete lattice is discussed. Results are given for the particular example of a model fcc crystal with (100) surfaces and anisotropy ratio η>1. The lattice results extend and complement the elastic continuum results of Lim and Farnell. General arguments allow qualitative extension of the present results to encompass the cases of cubic crystals having different surfaces [viz., (110) or (111)] or having anisotropy ratio η<1.

Surface phonon frequencies for magnesium oxide

Abstract Surface phonon branches and the surface excess distribution of frequencies f S (ω) have been calculated for the (001) face of MgO with a rigid shell model for a fifteen-layer film. Comparison with the f S (ω) obtained by Rieder and Horl from neutron scattering shows agreement with some features as well as the need for further experimental work.

On the existence of long-wavelength shear-horizontal surface waves on the bcc (001) surface☆

Abstract Long-wavelength, low-frequency shear-horizontal surface waves are found to exist along 〈100〉, but not 〈110〉, directions of propagation on the (001) face for a simple but fairly realistic model of monatomic body-centered cubic crystals. This domain of existence contrasts with that found previously on the (001) face of monatomic simple cubic and both monatomic and rocksalt face-centered cubic structures, where such waves exist along 〈110〉, but not 〈100〉.

On the importance of sp-electrons in the surface relaxation of d-band metals☆

Abstract We present arguments to the effect that the self-consistent smoothing of the mobile sp-electron distribution gives rise to electrostatic forces which are a major contribution to the initial relaxing forces on the outermost ions at a d-band metal surface. Numerical estimates for the (001) face of α-Fe are given, showing that for the surface ions the surface Madelung force from the sp-electrons is inward and about 3 1 2 times stronger than an outward short-range force simulating the effect of d-electrons.