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

Raman scattering and ³¹P NMR results show that the backbone of binary PₓSe_1-x glasses is composed of Seₙ-chain fragments, pyramidal P(Se1/2)₃ units, quasitetrahedral Se=P(Se1/2)₃ units, and ethylenelike P₂(Se1/2)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 Tg(x) from agglomeration theory are compared to the observed Tg(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 $x(0.12 0.47),$ a rapid phase separation of monomeric P₄Se₃ units from the backbone takes place, leading to a molecular glass with a rather low Tg at $x>0.50.$