Structural Transformation in Ge_xS_100−x (10 ≤ x ≤ 40) Network Glasses: Structural Varieties in Short-Range, Medium-Range, and Nanoscopic Scale

Precise x-ray diffraction measurements using high-energy x rays of synchrotron radiation and systematic Raman scattering measurements were carried out for Ge_xS_100−x (10 ⩽ x ⩽ 40) network glasses. The structural models of the network glasses were proposed based on the results. In the stoichiometric composition Ge₃₃S₆₇, GeS₄ tetrahedral units are connected forming either corner-sharing or edge-sharing structures. In the S-rich glasses, S atoms are inserted between two neighboring GeS₄ tetrahedra, resulting in a flexible floppy network. In a much more S-rich region, some S₈ ring molecules are isolated from the network, and assemble to form a crystal in nanoscopic scale. In this respect, Ge₁₀S₉₀ samples are regarded as crystallized glasses. In the Ge-rich region, the GeS₄ tetrahedra are connected with bridging Ge atoms. The connection makes a new rigid network. The bridging Ge-S bond is weaker than the intratetrahedron bond, and this leads to drastic changes in the optical properties.