Investigation of the peculiarities in the polarized reflectance spectra of some Tutton salt monoclinic single crystals using dispersion analysis

Dispersion analysis in the ν3(SO42−) frequency region of two Tutton salt monoclinic single crystals, K2Ni(SO4)2·6H2O and (NH4)2Ni(SO4)2·6H2O, was performed. Employing analytical expressions for the dielectric tensor and the reflectance function valid for monoclinic crystals, oscillator parameters like, transversal mode frequencies and the corresponding attenuation constants, transition moments orientation and oscillator strengths as well as longitudinal mode frequencies and the corresponding transition moment orientations were obtained. These oscillator parameters were used in simulations in order to explain certain peculiarities in the reflectance spectra occurring with directional changes of the polarization vector. From the simulations it follows that the appearance of spectra with two totally overlapped bands (having quite different oscillator strengths) depends particularly on the orientation of the longitudinal transition moment of the mode with the smaller LO–TO splitting.