Infrared vibrational Modes in Tl3SbS3, Tl3AsS3, and Tl3AsSe3 crystals

Reflectivity spectra of Tl₃SbS₃, Tl₃AsS₃, and Tl₃AsSe₃ crystals have been investigated in the wave number range 50–600 cm^?1 for the polarizations E∥c and E?c. The fundamental phonon parameters, the limiting dielectric constants ?_∞ and ?_S and the reflectivity spectra contours have been calculated by using classical dispersion relations for both E∥c and E?c configurations. The Szigeti effective charges and the relative ion charges of As, Tl, Sb, Se, S anions and cations have been calculated in dependence on the incident light polarization.