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

Abstract

Raman scattering and ${}^{31}\mathrm{P}$ NMR results show that the backbone of binary ${\mathrm{P}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ glasses is composed of ${\mathrm{Se}}_{n}$-chain fragments, pyramidal ${\mathrm{P}(\mathrm{S}\mathrm{e}}_{1/2}{)}_{3}$ units, quasitetrahedral ${\mathrm{S}\mathrm{e}=\mathrm{P}(\mathrm{S}\mathrm{e}}_{1/2}{)}_{3}$ units, and ethylenelike ${\mathrm{P}}_{2}{(\mathrm{S}\mathrm{e}}_{1/2}{)}_{4}$ 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 ${\mathrm{P}}_{4}{\mathrm{Se}}_{3}$ units from the backbone takes place, leading to a molecular glass with a rather low ${T}_{g}$ at $x>0.50.$