Molecular Origin of Glass Forming Tendency in Ternary Te-Se-Br(Cl) Chalcohalide Glasses

Abstract

Abstract Bulk glasses of Te3 _zSez-(CI or Br)2+y , the composition spanning the glass forming range, were characterized by differential scanning calorimetry, 125Te absorption, and 129I emission Mössbauer spectroscopy. The results reveal that the stoichiometric glasses Te3(Cl or Br)2 consists of c-Te3 Cl2 -like chain fragments about 1.5 nm long and terminated by one-fold coordinated halogen atoms. The 125Te electric hyperfine structure results reveal that Te replacement by Se (z ≠ 0) leads to preferential occupancy at those Te sites in the chains that are two-fold coordinated, and in a rather striking fashion demonstrate that the average length of the chain fragments remains independent of Se concentration. Halogen atoms in the glasses act as chain terminators, and also bond to Te-sites in the chains to produce four-fold coordinated sites. The low glass transition temperatures (Tg ~ 70 -90°C), the low average coordination number of the proposed chain-fragments, along with the hyperfine structure results, suggest that the extensive glass forming tendency in the chalcohalides derives from the floppiness of the chain-fragments.