Structure and delocalized vibrational modes in vitreousSix(Se1−yTey)1−x

Vitreous samples of ${\mathrm{Si}}_{\mathrm{x}}$(${\mathrm{Se}}_{1\mathrm{-}\mathrm{y}}$ ${\mathrm{Te}}_{\mathrm{y}}$ ${)}_{1\mathrm{-}\mathrm{x}}$, 0.333≤x≤0.43, were synthesized by melt quenching, and analyzed by Raman spectroscopy. The results support the cross-linked chain-cluster (CLCC) model proposed for ${\mathrm{Si}}_{\mathrm{z}}$ ${\mathrm{Se}}_{1\mathrm{-}\mathrm{z}}$ glasses. Tellurium tends not to substitute for selenium in the chains of edge-sharing ${\mathrm{SiSe}}_{4/2}$ tetrahedra. It does substitute at corner-sharing Si(Se,Te${)}_{4/2}$ sites that cross link the individual chains, and prefers the chain-terminating ${\mathrm{Si}}_2$(Se,Te${)}_{6/2}$ locations. Different aspects of medium-range order that arise from coupling of similar structural units are discussed in the context of the CLCC model. Also presented is an alternative interpretation of observed Raman bands in terms of delocalized vibrational excitations spanning over such coupled structures.