The glass formation region of the system and the vitreous‐crystalline transformation were investigated. Electrical and optical properties in the vitreous and in the polycrystalline state were studied as a function of composition on homogeneous and geometrically well defined samples prepared by a new method. Activation energies obtained from electrical and optical measurements were in excellent agreement and were found to decrease with increasing content of . For a given composition, the activation energy in the vitreous state was greater than in the crystalline state by 0.2–0.4 eV. Excepting the absorption edge, no absorption or reflection bands were observed in all of the compositions studied in the ranges of 0.7–1.7 and of 2.5–50µ.