Crystalline Si${\mathrm{Se}}_2$ and ${\mathrm{Si}}_x{\mathrm{Se}}_{1-x}$ glasses: Syntheses, glass formation, structure, phase separation, and Raman spectra

Crystalline Si${\mathrm{Se}}_2$ and a series of ${\mathrm{Si}}_x{\mathrm{Se}}_{1-x}$ clear yellow-orange glasses ($0.33<x\le 0.4$) were synthesized and studied by Raman spectroscopy. Spectra of crystals were analyzed in terms of the known crystal structure of Si${\mathrm{Se}}_2$ whereas for the glasses a new cross-linked chain-cluster model (CLCC) is proposed and is the basis for extensive spectral analysis. Extended chain-only and random-network models are inconsistent with the data. The CLCC model is characterized by well-defined chains of edge-sharing (Si${\mathrm{Se}}_4$) tetrahedra, as in the crystal, which are cross-linked via corner-sharing (Si${\mathrm{Se}}_4$) tetrahedra to form very large, highly viscous inorganic polymers. The structure has extensive medium-range order and provides for a self-consistent mechanism for transition to nonstoichiometric glasses in the range Si${\mathrm{Se}}_2$ to ${\mathrm{Si}}_2$ ${\mathrm{Se}}_3$. The smooth transition to various compositions is readily monitored through spectral changes in the Si-Se vibrational stretching region (150—300 ${\mathrm{cm}}^{-1\mathrm{}}$) and in the region of the collective modes at lower frequency (0—150 ${\mathrm{cm}}^{-1\mathrm{}}$).