Optical phonon modes and infrared dielectric properties of monoclinic CoWO sub(4) microcrystals

• Published in: Journal of physics. D, Applied physics Vol. 49; no. 4; pp. 45305 - 45314
• Main Authors: Moreira, Roberto L, Almeida, Rafael M, Siqueira, Kisla P F, Abreu, Cintia G, Dias, Anderson
• Format: Journal Article
• Language: English
• Published: 01.02.2016
• Subjects: Dielectrics; Infrared spectroscopy; Mathematical analysis; Mathematical models; Microcrystals; Phonons; Polarization; Tensors
• ISSN: 0022-3727; 1361-6463
• DOI: 10.1088/0022-3727/49/4/045305

The phonon characteristics of CoWO sub(4) 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 (A sub(u) modes) and along several angles within the ac-plane (B sub(u) 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 B sub(u) 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 mu m), 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 CoWO sub(4) crystals, with their correct symmetries attributed.