ON THE INTERATOMIC CORRELATIONS AND MEAN-SQUARE RELATIVE ATOMIC DISPLACEMENTS IN AN ANHARMONIC MODEL OF THE CLOSE-PACKED CRYSTAL

The correlative method of unsymmetrized self-consistent field (CUSF) i s used to study the interatomic correlations and mean square displacem ents in anharmonic crystals. In the first order of CUSF we have derive d the formula for the atomic mean square displacement and quadratic co rrelation between displacements of two nearest neighbors along the lin e passing through their centers C-xx(1) = nq(x)(2)/Z where n is the di mensionality of a lattice and Z is the coordinational number. In the s econd order of CUSF we have calculated the quadratic correlation momen ts and mean square relative displacements in the two-dimensional model of an anharmonic crystal with hexagonal lattice. The results are comp ared with those for square lattice obtained previously.