Compositional Evolution of the Structure and Connectivity in Binary P–Se Glasses: Results from 2D Multinuclear NMR and Raman Spectroscopy

Abstract

The atomic structure of binary P x Se 100−x glasses with 5 ≤ x ≤ 70 is investigated using Raman spectroscopy and twodimensional 77 Se and 31 P isotropic/anisotropic correlation nuclear magnetic resonance (NMR) spectroscopy. These spectroscopic results, when taken together, demonstrate that the structure of P x Se 100−x glasses with x ≤ 50 consists primarily of −Se−Se−Se− chain elements, pyramidal P(Se 1/2 ) 3 units, ethylene-like 2/2 SeP− PSe 2/2 units, and Se=P(Se 1/2 ) 3 tetrahedral units. The chain structure of Se becomes increasingly cross-linked by P−Se polyhedral units, and the degree of connectivity increases with a progressive increase in P content up to x ∼ 50, at which point the −Se−Se−Se− chain elements completely disappear, and the structure becomes highly rigid. The compositional variation of the Se−Se−Se environments as obtained from the 77 Se isotropic NMR spectra reveals that the connectivity between the Se−Se and P−Se units in glasses with x ≤ 50 is intermediate to that of the random and the fully clustered scenarios. A further increase in P content results in the formation of P 4 Se 3 molecules such that at x = 63, the structure becomes predominantly molecular, consisting of P 4 Se 3 molecules likely held together via van der Waals forces. The structure of glasses with x > 63 is characterized by P 4 Se 3 molecules and likely nonmolecular P 4 Se 3 -like species, along with amorphous red phosphorus-like regions. These P 4 Se 3 -like moieties and the amorphous red phosphorus-like units can connect to each other via P−P bonds, and their relative concentrations increase with increasing P content. This compositional evolution of structural connectivity of P x Se 100−x glasses is shown to be consistent with the corresponding variation in the glass transition temperature.