Raman scattering and ${}^{31}\mathrm{P}$ NMR results show that the backbone of binary ${\mathrm{P}}_x{\mathrm{Se}}_{1-x}$ 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 $\mathrm{}(x<\mathrm{}0.47\mathrm{}).$ Concentrations of the various building blocks independently established from each spectroscopic probe are found to be correlated. Theoretical predictions for the glass transition variation $T_g\mathrm{}\left(x\right)\mathrm{}$ from agglomeration theory are compared to the observed $T_g\mathrm{}\left(x\right)\mathrm{}$ trends established from temperature-modulated differential scanning calorimetry. The comparison shows that a stochastic network description is an appropriate one of glasses at low $x\mathrm{}(x<\mathrm{}0.12\mathrm{}).$ At medium $x\mathrm{}(0.12<x<\mathrm{}0.47\mathrm{}),$ substantial medium-range structure evolves in the form of polymeric ethylenelike units that comprise elements of the barely rigid backbone. At higher $x\mathrm{}(x>\mathrm{}0.47\mathrm{}),$ 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 $x>\mathrm{}\mathrm{0.50.}\mathrm{}$

• Received 22 February 2000

DOI:https://doi.org/10.1103/PhysRevB.64.134204