Effect of Annealing of Ca3TaGa3Si2O14 Catangasite Crystals on Their Optical Activity

The spectral dependences of transmission and absorption in the wavelength range of 200–2500 nm were measured for Ca₃TaGa₃Si₂O₁₄ crystals cut perpendicular to the optical axis in the initial state (without annealing) and after isothermal annealing in vacuum and in air. It was found that annealings in vacuum and air lead, respectively, to decrease and increase in the absorption band intensity. A spectrophotometric method for measuring and calculating the specific rotation angle ρ of the plane of polarization of light in gyrotropic crystals from the transmission coefficient spectra at different angles between the polarizer and analyzer is considered. The transmission spectra were normalized to eliminate the spectral shifts associated with measurement features. Spectral dependences of ρ, approximated by the extended Drude formula, were obtained for all three samples; the influence of the annealing atmosphere on the coefficients of this formula is established.