Molecular structure, glass transition temperature variation, agglomeration theory, and network connectivity of binary P-Se glasses

Raman scattering and P-31 NMR results show that the backbone of binary PxSe1-x glasses is composed of Se-n-chain fragments, pyramidal P(Se-1/2)(3) units, quasitetrahedral Se=P( Se-1/2)(3) units, and ethylenelike P-2,(Se-1/2)(4) units at low P content (x<0.47). Concentrations of the various building blocks independently established from each spectroscopic probe are found to be correlated. Theoretical predictions for the glass transition variation T-g(x) from agglomeration theory are compared to the observed T-g(x) trends established from temperature-modulated differential scanning calorimetry. The comparison shows that a stochastic network description is an appropriate one of glasses at low x (x<0.12). At medium x (0.12<x<0.47), substantial medium-range structure evolves in the form of polymeric ethylenelike units that comprise elements of the barely rigid backbone. At higher x (x>0.47), a rapid phase separation of monomeric P4Se3 units from the backbone takes place, leading to a molecular glass with a rather low T-g at x>0.50.