Optical phonon modes and infrared dielectric properties of monoclinic CoWO 4 microcrystals.

Abstract

The phonon characteristics of CoWO4 microcrystals with monoclinic Wolframite structure were investigated by far-infrared (IR) and Raman spectroscopies. Near-normal spectra were taken for IR light polarization along the principal b-axis (Au modes) and along several angles within the ac-plane (Bu modes). The IR spectra were analyzed with a generalized Drude– Lorentz model, and all predicted polar phonon modes were fully determined, including their symmetries, the dielectric Lorentz parameters and the non-orthogonal phonon polarizations for the Bu modes. Anomalous dispersion and negative values for the real and imaginary parts of the off-diagonal components of the dielectric tensor functions were identified and discussed under the light of the varying phonon polarization directions (spread out in the ac-plane). The obtained static and background dielectric tensors gave an average permittivity of 16.1 (at microwave region), refractive indices along the principal dielectric axes of 2.22, 2.33 and 2.44 (at 1 μm), the optical axes, and an estimated value for the biaxial angle of the crystal. Polarized Raman spectra on appropriate scattering configurations revealed the 18 non-polar gerade phonons of CoWO4 crystals, with their correct symmetries attributed.