Mechanism of Formation of Semiconductor Glass–Ceramics during Bulk Isothermal Crystallization of As2Se3Sn0.4 Glass

119Sn nuclear gamma-resonance spectroscopy (119Sn NGRS), X-ray diffraction (XRD) and long-wavelength IR spectroscopy, as well as measurements of density, microhardness, and electrical conductivity parameters of quenched samples, were used to study step-by-step kinetically coupled structural–chemical and phase transformations during bulk isothermal crystallization of As2Se3Sn0.40 (7.4 at % Sn) semiconductor glass at 240°C, leading to the formation of electrically conductive, chemically resistant glass–ceramics. Analysis using the generalized Kolmogorov–Avrami equation showed that, homogeneous nucleation and two-dimensional growth of finely dispersed SnSe and SnSe2 crystals occur at the first stage of reconstructive crystallization at a temperature of 240°C, with the predominance of the SnSe phase, which initiates heterogeneous nucleation and two-dimensional growth of crystals of the main As2Se3 phase at the second stage. Changes in vibrational spectra and electrical conductivity parameters at different stages of bulk crystallization of As2Se3Sn0.40 glass at 240°C are discussed. The kinetics of isothermal crystallization of glass at the first stage are also studied using continuous conductometry at temperatures of 240 and 260°C.