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

The atomic structure of binary PSe_100- glasses with 5 ≤ x ≤ 70 is investigated using Raman spectroscopy and two-dimensional ⁷⁷Se and ³¹P isotropic/anisotropic correlation nuclear magnetic resonance (NMR) spectroscopy. These spectroscopic results, when taken together, demonstrate that the structure of PSe_100- glasses with x ≤ 50 consists primarily of -Se-Se-Se- chain elements, pyramidal P(Se_1/2)₃ units, ethylene-like _2/2SeP-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 ⁷⁷Se 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 P₄Se₃ molecules such that at x = 63, the structure becomes predominantly molecular, consisting of P₄Se₃ molecules likely held together via van der Waals forces. The structure of glasses with x > 63 is characterized by P₄Se₃ molecules and likely nonmolecular P₄Se₃-like species, along with amorphous red phosphorus-like regions. These P₄Se₃-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_100- glasses is shown to be consistent with the corresponding variation in the glass transition temperature.