Assignment of Terahertz Modes in Hydroquinone Clathrates

Hydroquinone (HQ) and its clathrate are materials of growing interests, due to their promising applications in energy related science and industries. Recently, many studies have been performed to understand the properties of these materials. Terahertz (THz) spectroscopy is a powerful tool for studying the properties of these materials by non-destructively probing their low-energy (meV) dynamics. Although terahertz spectra of HQ and its clathrates have been measured, a report on the correspondence between THz spectra and low frequency dynamics is still lacking. In this paper, we measure the temperature-dependent THz spectra of both α-HQ (the non-clathrate form) and β-HQ clathrate, with Ar and CO2 as guest species. We also perform density functional theory (DFT) simulations on these materials, in order to assign the spectral features. We find an excellent match between the experimental and the DFT calculated spectra. Using the simulation result, we build connections between the THz spectra and the atomic motions in these materials. In addition, we also perform DFT simulations on β-HQ-He, β-HQ-Ne, and β-HQ-Kr to study the patterns in the change of THz spectra as the guest species changes.