Physical Properties Evolution in Network Glasses of Ge–Cd–Se System: A Theoretical Approach

Abstract

In technology, Ge-Se-Cd is a prototypical chalcogenide system capable of
forming glasses on various components. A theoretical analysis was conducted to
determine the effect of Ge-addition on the physical parameters of GexCd10Se90-x (x=5,
10, 15, 20 and 25 at. %) glasses systems. The values of the average coordination
number, the number of constraints, the average heat of atomization, cohesive energy,
density, and packing density were increased with an increase in the Ge content, while
the values of lone pair electrons were decreased as the system was moving towards the
rigid region. Deviation of the stoichiometry values leads the system to chalcogen-rich
regions, except for Ge25Cd10Se65 indicating chalcogen-poor composition. A linear
correlation was found between the overall mean bond energy and glass transition
temperature. Increasing the Ge content leads to an increase in strong heteropolar Ge-Se
bonds at the expense of weak homopolar Se-Se bonds, resulting in an increase in
network connectivity or the rigidity of the system. The chemical order network model
(CONM) was applied to calculate the distribution of the chemical bonds and the
cohesive energy of the system. According to the Neffati relation, the theoretical band
gap has been calculated. The increase in the energy gap with increasing Ge
concentration for all systems except Ge25Cd10Se65 suggests that the 2D layered structure
generated by the branching of the Se chain is completely established at this composition
at <r>=2.6, after that point, the layer structures cross-link, forming a rigid 3D network.
Such changes can broaden the lone-pair valence band, reducing the band gap. Physical
parameters were sensitive to changes in composition subsequently making this present
system suitable for phase change optical recording.