Optical zone-centre phonon modes and macroscopic elasticity in hcp metals

A simple relation between zone-centre optical phonon modes and elastic constants for hcp metals derived previously from lattice dynamical force constant models using central and angular interactions is found to be reasonably valid for elemental metals with low elastic anisotropy. Application of this formalism to high-pressure phonon frequency data implies that phase transitions from hcp to Sm-type and to the ω-phase in the regular lanthanides and Zr, respectively, involve softening of the elastic constant C₄₄, and that the rapid decrease under compression of an `unusual' high ambient-pressure TO mode Gruneisen parameter in Zn is related to the gradual transition from anisotropic→isotropic compressional behaviour with increasing pressure.