Microscopic Origin of the Glass Forming Tendency in Germanium-Tin or SELENIUM)(3) Glassy Networks.

The glass forming tendency in the ternaries Ge _{rm 2-2x}Sn _{rm 2x}(S or Se) _3 has been systematically studied as a function of composition (x) using Mossbauer spectroscopy, Raman scattering, scanning calorimetry, and molar volumes. The experimental results demonstrate that the glass forming tendency near x ~eq 0.5 in both the S- and the Se- containing glasses is the result of a particular molecular fragment present in the corresponding networks. This fragment is based on the open zig-zag chain structure of crystalline GeSnS_3. Specifically, the structure consists of covalently bonded chains of Ge(S _{1/2})_4 tetrahedra laterally coupled by weaker Sn-S bonds. It is determined that the openness and flexibility or floppiness of this covalently bonded structure promotes glass formation. The major structural difference between GeSnS_3 and GeSnSe_3 is that the Se-containing crystal, unlike the S-containing one, does not exist as a stoichiometric crystal. Instead, c-GeSnSe _3 phase separates into a multitude of structures. These experimental results demonstrate that, while GeSnS_3 glass conforms to the Ioffe-Regel Rule, GeSnSe_3 is a novel exception to the rule.