Covalent bond connectivity, medium range order, and physical properties in TeX and TeXAs glasses

Data on the new As-Te-Se-I glasses are analyzed to determine whether the extended order determined by bond connectivity via the average coordination number, 〈r〉, controls the physical properties as it does in the case of the Ge-As-Se system. When appropriate allowance is made for partial 4-coordination of Te in Te- and I-rich glasses, the calorimetric glass transition temperature T_g is found to be a universal function of 〈r〉 as in Ge-As-Se (4-3-2) glasses. However, no special behavior is found at the Phillips-Thorpe rigidity percolation threshhold value, 〈r〉=2.4 as in the 4-3-2 case. The 〈r〉-dependences of both linear and non-linear aspects of relaxation appear smaller in the present system, and no extrema are in evidence. This is interpreted to indicate that the 2- vs 3-dimensional nature of crosslinking, by As vs. Ge, respectively, of the basic chain structures obtained at all 〈r〉=2 compositions, is an important property-controlling factor.