Ab initio Investigation of Structural Units and Raman Vibrational Characteristics in Ge-Se-Te Glasses

Chalcogenide glasses in the Ge-Se-Te system possess wide infrared transparency and strong optical nonlinearity, yet the microscopic origin of their vibrational behavior remains unclear. Using ab initio calculations, we analyzed Raman-active modes in GeSexTe4−x (x = 0–4) tetrahedra, edge-sharing tetrahedra, and ethane-like Ge2Se2xTe6−2x (x = 0–3) clusters. For GeSexTe4−x (x = 0–4) tetrahedra, the symmetric stretching vibrations exhibit two families: Ge-Se-dominated and Ge-Te-dominated modes, both showing monotonic redshifts as the number of same-type bonds increases. In edge-sharing tetrahedra, the Ge-Ch-Ge-Ch (Ch = Se or Te) four-membered-ring breathing frequency decreases with higher Te content, and a comparable softening is observed in the A1g and Eg modes of ethane-like Ge2Se2xTe6−2x units, where Te substitution lowers the breathing and antisymmetric stretching frequencies. These systematic redshifts are well explained by harmonic vibrational theory and bond polarizability models, indicating that Te substitution increases effective mass, softens local force constants, and redistributes bond-polarizability contributions, collectively leading to weaker Ge–Ch bonding and reduced structural rigidity. The results provide microscopic insight into the compositional evolution of Raman features in Ge-Se-Te chalcogenide glasses.