PHONON DISPERSIONS IN CALCIUM TUNGSTATE

Abstract
External mode formalism is applied to study phonons in CaWO₄. The effective ionic charges of the Coulomb interactions and the effective ionic radii of Born-Mayer shortrange potential are determined so that the dynamical equilibrium conditions are satisfied and lattice energy is of right order in comparison with other complex ionic crystals. The calculated phonon dispersion relations along [001] and [110] directions are in reasonable agreement with the neutron data. The generalised LST relations computed with our model agree with the IR data. Zone centre phonons in few other scheelites are also investigated and compared with experimental data to study the influence of polarizability and a possible breakdown of external mode formalism.