Topological Origin of the Super-Flexible Phase of Se and Se-Rich Glasses and Aging-Induced 5-fold Narrowing of Glass Transition Width

Pure Se glass and ternary A(x)B(y)Se(1-x-y) glasses where A = Ge and B = P or As, in the low mean coordination number, ⟨r⟩, range of 2.00 < ⟨r⟩ < 2.12 display a 3- to 5-fold reduction in the width of the glass transition when aged at room temperature over 4-8 months. Group IV (Ge) and group V (P, As) additives serve to crosslink the base Se glass polymeric chains, Se-n, with n > 250 atoms and to reduce the length "n" between cross-link points as x and y are increased. Herein, it is shown that in such weakly crosslinked glasses, the continued narrowing of the glass transition width T-g by a factor of 3-5, as in pure Se, stems from the fact that polymeric Se-n chain segments between the crosslink points continue to have a length n of at least eight atoms or more. Such polymeric Se-n chains are super-flexible and reconstruct with each, as in pure Se, promoting structural ordering responsible for T-g narrowing. When n < 8, super flexibility is steadily lost. Along with the flexible, intermediate, and stressed-rigid phases, a new super-flexible phase of Se and Se-rich glasses is obtained.