MOLECULAR-ORIGIN OF GLASS-FORMING TENDENCY IN TERNARY TE-SE-BR(CL) CHALCOHALIDE GLASSES

Bulk glasses of Te3-zSez (Cl or Br)(2+y), the composition spanning the glass forming range, were characterized by differential scanning calo rimetry, Te-125 absorption, and I-129 Mossbauer spectroscopy. The resu lts reveal that the stoichiometric glasses Te-3(Cl or Br)(2) consists of c-Te3Cl2-like chain fragments about 1.5 nm long and terminated by o ne-fold coordinated halogen atoms. The Te-125 electric hyperfine struc ture results reveal that Te replacement by Se (z not equal 0) leads to preferential occupancy at those Te sites in the chains that are two-f old coordinated, and in a rather striking fashion demonstrate that the average length of the chain fragments remains independent of Se conce ntration. Halogen atoms in the glasses act as chain terminators, and a lso bond to Te-sites in the chains to produce four-fold coordinated si tes. The low glass transition temperatures (T-g similar to 70 - 90 deg rees C), the low average coordination number of the proposed chain-fra gments, along with the hyperfine structure results, suggest that the e xtensive glass forming tendency in the chalcohalides derives from the floppiness of the chain-fragments.