Processing Gray Selenium in Phosphonium-Based Ionic Liquids.

The dissolution of gray selenium in tetraalkylphosphonium acetate ionic liquids was investigated by UV-vis, NMR, and Raman spectroscopy as well as quantum chemical calculations and electrochemical methods. Acetate anions and tetraalkylphosphonium cations facilitate the formation and stabilization of oligoselenides Se _n ^2- and cationic Se species in the ionic liquid phase. Chemical exchange of selenium atoms was demonstrated by variable-temperature ⁷⁷ Se NMR experiments. Additionally, uncharged cycloselenium molecules exist at high selenium concentrations. Upon dilution with ethanol, amorphous red selenium precipitates from the solution. Moreover, crystalline Se _{1- x} Te _x solid solutions precipitate when elemental tellurium is added to the mixture as confirmed by powder X-ray diffraction and Raman spectroscopy.