Variation of glass transition temperature, Tg, with average coordination number, 〉 m〈, in network glasses: evidence of a threshold behavior in the slope |d Tg/ d〉 m〈 | at the rigidity percolation threshold (〉 m〈 = 2.4)

Abstract

Alkali oxide (Ak 2O) addition to telluria lowers glass transition temperature, T g, of (Ak 2O) x (TeO 2) 1− x glasses systematically, with the slope d T g/d x displaying a local maximum at x c ≅ 0.18 corresponding to 〉 m〈 ≅ 2.4, the rigidity percolation threshold. In covalent network glasses, as in the present alkali tellurate glasses, T g is found to increase with 〉 m〈 with the slope |d T g/d〉 m〈| displaying a maximum near 〉 m〈≅2.4. It is recognized that this threshold behavior can be traced to a qualitative increase of molecular relaxation time near 〉 m〈≅2.4, where a condition for mechanical equilibrium is locally satisfied. This increase leads to a local T g(〉 m〈) enhancement at 〉 m〈 = 2.4 due to a kinetic effect, which is superposed on a quasi-linear T g(〉 m〈) variation with 〉 m〈 due to chemical effects.