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

A lattice dynamical model is developed by applying the Clark-Gazis-Wal lis-type three-particle forces in the hexagonal close-packed (hcp) sys tem. The model is then applied in the study of the phonon dispersion i n four hcp metals, namely, Be, Tb, Ho, and Lu. The present scheme give s the dynamical matrix element Im D12 (q, 11) to be nonzero and explai ns the peculiar behaviour of dispersion branches along GAMMAKM directi on in these hcp metals. Further more, a single expression for C44 is f ound and the Cauchy discrepancy (C-13-C44) is entirely ascribed to the three-particle forces.