Boson peak in the room-temperature molten salt tetra(n-butyl)ammonium croconate

Low-frequency (5–200 ${\mathrm{cm}}^{\mathrm{-}1}$) Raman spectra of the room-temperature molten salt, tetra(n-butyl)ammonium croconate (TBCR), $\left[\right(n-{\mathrm{C}}_4{\mathrm{H}}_9{)}_4{\mathrm{N}]}_2{\mathrm{C}}_5{\mathrm{O}}_5\cdot {4\mathrm{H}}_2\mathrm{O},$ are reported. TBCR is a very viscous liquid with glass transition temperature $T_g=20\mathrm{}°\mathrm{C}.$ According to the temperature dependence of the viscosity, TBCR has been classified as a fragile glass-forming liquid. Raman spectra of TBCR have been obtained from 360 K down to a deep supercooled state at 77 K. The boson peak is observed in the Raman spectra as long as the system is cooled below $T_g.$ The spectra are discussed in terms of proposed theories for depolarized light scattering in glass-forming liquids.