at. %) have been synthesized. Neutron and X-ray diffraction techniques were used to study the atomic glassy structure, and Reverse Monte Carlo (RMC) simulations were applied to model the 3-dimensional atomic configurations and thorough mapping of the atomic parameters, such as first and second neighbour distances, coordination numbers, and bond-angle distributions. The results are explained with formation of GeSe 4 and SbSe 3 structural units, which correlate with the Ge/Sb ratio. For all the studied compositions, the Ge-Se, Sb-Se, Ge-Ge, and Se-Se bonds are significant. RMC simulations reveal the presence of Ge-Sb and Sb-Sb bonds, being dependent on Ge/Sb ratio. All atomic compositions satisfy formal valence requirements, i.e., Ge is fourfold coordinated, Sb is threefold coordinated, and Se is twofold coordinated. By increasing the Sb content, both the Se-Ge-Se bonds angle of 107±3 ∘ and Se-Sb-Se bonds angle of 118±3 ∘ decrease, respectively, indicating distortion of the structural units. Far infrared Fourier Transform spectroscopic measurements conducted in the range of 50-450 cm -1 at oblique (75 ∘ ) incidence radiation have revealed clear dependences of the IR band's shift and intensity on the glassy composition, showing features around x=27 at.% supporting the topological phase transition to a stable rigid network consisting mainly of SbSe 3 pyramidal and GeSe 4 tetrahedral clusters. These results are in agreement with the Reverse Monte Carlo models, which define the Ge and Sb environment.

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Chalcogenide glasses Ge x Sb 40−x Se 60 with composition 12, 25 and 30 at.%, synthesized from elements with 5N purity (Ge, Sb, Se) by the conventional melt-quenching method were studied. The spectrophotometer's measurements were recorded to determine the optical properties. Models based on the absorbance and absorption coefficient are used to determine the optical band-gap energy. The Urbach energy is deduced in the -Ge Sb Se x x 40 60 thin film alloys with composition x=12, 25 and 30 at.%. The electronic and structural properties are studied, and the influence of the Urbach energy in estimating of the nonlinearity in the optical properties of these glasses is discussed. The analysis is performed with respect to the Urbach parameter that quantifies the degree of crystallinity of the structure which can be used to understand the behavior of different diffractive, wave-transmission and fiber structures.