INFRA R ED SPECULAR REFLECTANCE SPECTRA OF PRESSED POLYCRYSTALLINE SAMPLES OF ALUMS − COMPARISON WITH SINGLE CRYSTAL SPECTRA

Vibrational properties of several alums were analyzed by employing polarized specular reflectance IR spectroscopy. A polycrystalline sample in a form of pellet was used for that purpose. Due to the cubic crystal system in which these crystals crystallize, the same equations for the reflectance and dielectric function were employed when conducting the dispersion analysis as those used for single crystals. Very good fit results were obtained for the investigated polycrystalline samples. Apart from this, dispersion analysis on single crystal reflectance spectra was also performed. Vibrational parameters, like the oscillator strength S, attenuation constant γ and the frequency of the transversal phonons ωt obtained from the fit of the polycrystalline samples, were compared with the corresponding data obtained from the dispersion analysis performed on single crystal specimens. The dielectric and optical properties of the alums were obtained and the LO–TO splitting for each of the modeled bands was calculated. Key words: alums; specular reflection spectroscopy; dispersion analysis; polycrystalline pressed pellets;  optical and dielectric propertie