The reflectivity spectrum of SbSeI crystals in the spectral range of 10 – 300 cm–1 over a wide range of temperatures (10 – 297 K) with light polarization E||c and Ec was experimentally studied. Measurements were carried out in Germany. Also experimental research was carried out with the reflectivity spectrum of SbSI crystals in the spectral range of 10-450 cm–1 with light polarization E||c over temperature range (273 – 350 K) . The spectra of optical constants and optical functions were calculated using the Kramers–Kronig (KK) technique and the method of optical parameter fitting (OP). Tables and diagrams are presented. Vibrational spectra of SbSeI crystals in harmonic approximation were theoretically investigated. The oscillation frequencies of normal vibrational modes and the amplitudes of normal coordinates along z(c) axis and in x-y plane by diagonalization of the dynamic matrix were calculated. Experimental studies of reflectivity spectrum of SbSeI crystals revealed, that there is no ferroelectric phase transition in SbSeI crystals in the temperature range of 10 - 297 K. And experimental studies of reflectivity spectrum of SbSeI crystals proved, that ferroelectric phase transition in SbSI crystals grown by us occurs at temperature TC = 293 K. SbSI crystals in range of temperatures T > TC are in paraelectric phase, and in range of temperatures T < TC are in ferroelectric phase. It was experimentally proved, that low-frequency IR mode of SbSeI, when E ׀׀ c is... [to full text]