Theoretical modeling of network topology, rigidity, and electronic structure in quaternary Se₆₅Ge₁₄₊₁₋ₓSb₂₀Teₓ chalcogenide glasses

Abstract

Chalcogenide glasses based on Ge–Se–Sb–Te systems are of great interest for advanced photonic and memory device applications due to their tunable optical, thermal, and structural properties. These glasses offer exceptional infrared transparency, high refractive index, and photo-induced switching behavior, making them promising candidates for phase-change memory and infrared optics. However, understanding how compositional variations, particularly the addition of tellurium (Te), influence network topology, rigidity, and electronic structure remains a key scientific challenge. In this work, a series of Se₆₅Ge₁₄₊₁₋ₓSb₂₀Teₓ glasses (x = 0, 1, 3, and 5 at%) were synthesized using the melt-quenching technique and systematically investigated through theoretical modeling and structural analysis. The aim was to explore how Te incorporation affects the average coordination number, mechanical constraints, cohesive energy, optical band gap, and thermal stability. Detailed calculations of constraint theory parameters (N_con, ⟨r_eff⟩), floppy mode fraction, lone-pair electron concentration, and bond energetics were performed to assess the topological and electronic transformations within the glass matrix. The results reveal that increasing Te content reduces the average coordination number and cohesive energy, indicating a softening of the glass network. At x = 5, the system reaches the isostatic threshold (N_con ≈ 3), maximizing network flexibility without compromising stability. This composition’s substantial increase in glass transition temperature (Tg ≈ 988 K) and mean bond energy (⟨E⟩ ≈ 4.12 eV/atom) suggests the formation of a thermally robust yet topologically optimized structure. Concurrently, the optical band gap narrows slightly, and the system retains high covalent character (> 99% in Te–Se bonds), ensuring desirable transparency and electronic performance.