Phonon dispersions in calcium tungstate

Abstract

In their studies on phonons in CaWO4, Kanamori et al. investigated the phonon dispersion of translational modes alone, ignoring the finite moment of inertia of tungstate ions and Coulomb interactions. In our present investigations, based on external mode formalism (Venkataraman and Sahni), we consider the coupling of the translational and librational modes and include the long range interactions. The effective ionic charges of the Coulomb interactions and the effective ionic radii of Born–Mayer short range potential arc determined so that the dynamical equilibrium conditions are satisfied and lattice energy is of the right order in comparison with other complex ionic crystals (Rao et al.). The phonon dispersion relations in CaWO4 along [001] and [110] directions arc calculated and found to be in reasonable agreement with available neutron data (Steinman et al.). The generalized LST relations (Venkataraman et al.) computed with our model agree with the IR data of Barker. In the long wavelength limit, the acoustic branches fit well with experimental data, indicating that this model can explain the acoustic properties as well. Zone centre phonons in CaMoO4, SrWO4, SrMoO4, BaWO4, BaMoO4, PbWO4, and PbMoO4 are also investigated and compared with experimental data to study the influence of polarizability and a possible breakdown of external mode formalism.