Dispersion analysis of polarized IR reflectance spectra of Tutton salts: The ν3(SO42−) frequency region

Polarized IR reflectance spectra of eight Tutton salts single crystals: M′2M″(SO4)2·6H2O; M′: K+ or NH4+; M″: Ni2+, Co2+, Zn2+ or Mg2+) were recorded at near-normal incidence. Dispersion analysis was performed for the whole mid-IR region. The results on ν3(SO42−) frequency region are reported in this paper. Reflectance spectra were recorded using radiation with polarization along the b crystallographic axis and in the ac crystal plane. Results of the fit for the transversal frequency, oscillator strength, attenuation constant and the orientation of the transition moments in the ac crystal plane for the ν3(SO42−) frequency region are presented. The longitudinal frequencies for the Au vibrations were obtained from the energy-loss function spectra, while for the Bu vibrations a particular approximation was used. In addition to frequency values, the directions of the longitudinal transition dipole moments in the ac crystal plane were also found. The dependence of the principal axes of the real and the imaginary parts of the dielectric tensor, on the radiation frequency, was established. An attempt was made to estimate the transition moment directions of the ν3(SO42−) mode components by correlation of the spectroscopic and structural findings.