Far infrared properties of SbSI-SbSeI mixed crystals

Far infrared reflectivity of eight SbS_1–x Se _x I mixed single crystals was measured in 50–400 cm^–1 region in polarized light at 25 and –170 C. While the low-frequency ferroelectric mode shows clear one-mode behaviour the second B_1u mode in 126–180 cm^–1 region shows two-mode behaviour. The existence of local and gap modes in this case is discussed.The author thanks ing. K.Nejezchleb from College of Chemical Technology in Pardubice for the crystals, L.Novák for helpful discussions, Z.Híová for computations and N.Dvoáková for help and figure drawing.