THE APPROXIMATION TO FREE-ENERGY OF THE ATOMS OF A MONOCRYSTAL SURFACE

Thermodynamic perturbation theory was applied to find the second-order term to the free energy approximation of the semiinfinite rectilinear atomic chain and the fee crystal with surface, whose interatomic pote ntial was described by the Morse function. The system of harmonic osci llators was chosen as the system of origin. The second order term was shown to be of the same order of magnitude as the first, and there is a temperature at which the chain, the square lattice and the fee latti ce lose their mechanical stability. There is a simple way to End the H elmholtz free energy of the system in this approximation. The expressi ons obtained were used to calculate the lattice constant, the mean-squ are displacement of an atom (MSD) and other parameters of the surface and the bulk of a monocrystal, and also the temperature dependencies o f these parameters. As temperature increases, surface relaxation (the displacement of the top layer) increases, while the anisotropy of the MSD of surface atoms changes slightly. The data on anisotropy of the M SD of surface atoms are in good agreement with available experimental results.