Quasiharmonic lattice dynamics of Bravais lattices. III. Thermal expansion of a rhombohedral lattice

For pt.II, see ibid., vol,7, no.18, 3269 (1974). The thermodynamic properties of a rhombohedral lattice model are studied in the quasiharmonic approximation. The model employs short-range central forces and can assume various forms of anisotropy. The Gruneisen functions and thermal expansion, under various extremes of anisotropy in the forces and geometry, are calculated as functions of temperatures. The results are analysed and explained in the light of a previous analysis of the individual normal mode frequencies and Gruneisen parameters. The behaviour of the mode is compared with that observed experimentally for a variety of axial crystals.