Abstract
The molecular structure of binary PxSe100-x glasses over the 0 < x < 57% range is examined in Raman scattering, Modulated Differential Scanning Calorimetry (MDSC) and Volumetric measurements. Raman scattering and trends in glass transition temperature T-g(x), provide evidence of two network backbones (BB) that are decoupled from each other, with T-g in the 0 < x < 40% range fixed by BB1 and in the 40% < x < 54% range by BB2. The 3D Backbone, BB1, is composed of Se-n chains cross-linked by P-centered pyramidal (PSe1/2)(3) units and Quasi-Tetrahedral [Se-P(Se-1/2) units and their 3-membered Pyramidal [(PSe1/2)(3)](3) ring, and Quasi Tetrahedral [Se-P(Se-1/2)(3)](3) ring counterparts. Backbone BB2, is composed of quasi-1D Ethylene-like P2Se2+x (x = 2,1,0) chains forming in the approximate 20% < x < 54% range. The enthalpy of relaxation at T g shows a square-well like variation in the 28% < x < 40% range determined by BB1, thus fixing the Flexible- and Intermediate- phase (IP). Compositional trends in melt Fragility index, m (x), show anomalously low value of 13.1 at x = 15% and of 7.7 at x = 34%. These minima in m(x) coincide with minima in the liquidus near the same compositions x and are traced respectively to the formation of the 3-membered rings and the decoupling of BB1 from BB2. 2021 Published by Elsevier B.V.