Deformable-ion model and lattice dynamics ofLiD7

Recently one of us (S.S.J.) showed that the deformation dipole model and the shell model are two special forms of a more general dipole model of lattice dynamics that follows rigorously from Zeyher's microscopic theory. Here we use a phenomenological approach to generalize the new model into a deformable-ion model (DIM) which is also consistent with Zeyher's microscopic theory. The DIM contains the long-range Coulomb interactions in the dipole approximation and it allows the rigid-ion short-range interactions to be modified by self-energy corrections due to symmetry-allowed deformations of the ions. We study the room-temperature phonon dispersion curves of ${}_{}{}^7{}_{}{}^{}\mathrm{LiD}$ to compare different forms of the DIM and in the process arrive at a simple form of DIM which is in very good overall agreement with dielectric, elastic, and neutron scattering data.