3-PARTICLE FORCES IN THE LATTICE-DYNAMICS OF SOME HEXAGONAL CLOSE-PACKED METALS

A lattice dynamical model is developed by applying the Clark-Gazis-Wallis-type three-particle forces in the hexagonal close-packed (hcp) system. The model is then applied in the study of the phonon dispersion in four hcp metals, namely, Be, Tb, Ho, and Lu. The present scheme gives the dynamical matrix element Im D12 (q, 11) to be nonzero and explains the peculiar behaviour of dispersion branches along GAMMAKM direction in these hcp metals. Further more, a single expression for C44 is found and the Cauchy discrepancy (C-13-C44) is entirely ascribed to the three-particle forces.