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

The atomic structure of binary PSe glasses with 5 ≤ ≤ 70 is investigated using Raman spectroscopy and two-dimensional Se and P isotropic/anisotropic correlation nuclear magnetic resonance (NMR) spectroscopy. These spectroscopic results, when taken together, demonstrate that the structure of PSe glasses with ≤ 50 consists primarily of -Se-Se-Se- chain elements, pyramidal P(Se) units, ethylene-like SeP-PSe units, and Se=P(Se) 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 ∼ 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 environments as obtained from the Se isotropic NMR spectra reveals that the connectivity between the Se-Se and P-Se units in glasses with ≤ 50 is intermediate to that of the random and the fully clustered scenarios. A further increase in P content results in the formation of PSe molecules such that at = 63, the structure becomes predominantly molecular, consisting of PSe molecules likely held together via van der Waals forces. The structure of glasses with > 63 is characterized by PSe molecules and likely nonmolecular PSe-like species, along with amorphous red phosphorus-like regions. These PSe-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 PSe glasses is shown to be consistent with the corresponding variation in the glass transition temperature.