Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 295

Since r(As) = 3 in the As_xSe_(1−x) system, r* = 2.4 corresponding to x* = 0.4. But good glass formation has been observed in the Se‐rich range from x ~ 0 to x ~ 0.23. As in the Ge–Se system, this discrepancy can be rationalized by viewing the As–Se glasses for x < 0.4 as a chemically ordered network made up of linearly‐rigid (Se_k) short chains, three of which being connected to every As atom. If one eliminates the internal degrees of freedom associated with the dihedral rotations of Se^# in the Se chains, it follows that these chemically ordered As((Se)_k)_3/2 systems are isostatic for x ≤ 0.4. Note that since x = 2/(2 + 3 k), k ≥ 2 corresponds to x ≤ 0.25, which fits well the reported composition range for good glass formation [28].