Chemical disorder in non-oxide chalcogenide glasses. Site speciation in the system phosphorus-selenium by magic angle spinning NMR at very high spinning speeds

D. Lathrop and H. Eckert

Abstract

The local structure of phosphorus-selenium glasses with P contents ranging from 5 to 75 atom % P is investigated by magic angle spinning (MAS) 31P NMR. In contrast to most oxidic glass systems, P-Se glasses require very high spinning speeds (12-14 kHz) for obtaining the resolution needed to differentiate between distinct sites present in these glasses. Chemical shift assignments are based on parallel solid-state NMR investigations on the crystalline reference compounds α-P4Se3, β-P4Se3, P4Se4, and α-P4Se3I2. The solid-state NMR spectrum of P4Se4 suggests that the molecular structure of this compound is not the one previously proposed on the basis of IR spectroscopy but is rather like the selenium analogue of α-P4S4. The results obtained on P-Se glasses confirm the prominent role of P4Se3 molecular constituents in glasses containing P contents ≥50 atom % but also show these units are absent at lower P contents. Below 35 atom % P, the MAS-NMR spectra are decidedly bimodal, indicating site differentiation between PSe3/2 and Se=PSe3/2 units. The compositional dependence of the peak area ratio is explained consistently in terms of a melt-equilibrium reaction between different short-range-order environments according to PSe3/2 + [Sen]1/n → Se=PSe3/2, with a phenomenological equilibrium constant K = 0.85 ± 0.05 (atom fraction)-1. The near-unity value of K reflects the efficient competition of homoatomic (Se-Se) versus heteroatomic (P=Se) bond formation in P-Se glasses, hence providing a rationale for the pronounced glass-forming tendency in this system.