Chemical bonding and the atomic structure ofSixSe1−xglasses

The nature of chemical bonding and the atomic structure of ${\mathrm{Si}}_x{\mathrm{Se}}_{1-x}$ glasses is investigated with the use of Raman spectroscopy and neutron-diffraction measurements. In these glasses heteropolar bonds are strongly favored at all compositions ($x$) and chemically ordered glassy compounds exist having the stoichiometries Si${\mathrm{Se}}_2$ and ${\mathrm{Si}}_2$ ${\mathrm{Se}}_3$. There is a one-to-one correspondence in the Raman modes of crystalline and glassy Si${\mathrm{Se}}_2$. Glassy Si${\mathrm{Se}}_2$ is composed of randomly oriented chains of edge sharing ${\mathrm{S}\mathrm{i}\left({\mathrm{Se}}_{\mathrm{½}}\right)}_4$ tetrahedra. Excess Se ($x<\mathrm{}\frac{1}{3}$) and excess Si ($x>\mathrm{}\frac{1}{3}$) are accommodated in the glassy structure as Se chains and ethanelike ${\mathrm{Se}}_3$-Si-Si-${\mathrm{Se}}_3$ units. The identifications of the various molecular units are made by comparison with Si-Br compounds and through the use of molecular-cluster calculations. The chemically ordered network model is a good description of the atomic structure of the glasses. A comparison of the present results is made with the previous work on Ge chalcogenide glasses. The differences are discussed in terms of the more covalent nature of Si and the greater molecular character of Si${\mathrm{Se}}_2$.