Far-infrared reflectivity spectra of thallium arsenic selenide crystal

Far infrared (IR) reflectivity spectra of thallium arsenic selenide (TAS) are measured for the first time, for both polarizations parallel and perpendicular to the c-axis, at 7, 100 and 250 K, in the frequency range 10-600 cm(-1). The phonon modes are classified within their symmetry species, their frequencies and oscillator strengths are calculated from the analysis of the experimental spectra. Splitting of several low-frequency optical modes indicates the possibility of a structural modification below 100 K. On the basis of an internal and external mode analysis and by comparison with similar compounds such as proustite and pyrargyrite, the origin of the various vibrational modes is discussed.