Assignment of Terahertz Modes in Hydroquinone Clathrates
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Assignment of Terahertz Modes in Hydroquinone Clathrates Wei Zhang 1 & Zihui Song 2 & Michael T. Ruggiero 2
& Daniel M. Mittleman
1
Received: 7 November 2019 / Accepted: 15 January 2020/ # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
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 nondestructively 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 temperaturedependent 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. Keywords Porous materials . Vibrational spectroscopy . Density functional theory . Lattice dynamics
1 Introduction Clathrates are a type of inclusion compound in which guest molecules are non-covalently trapped within a framework of a host species. In recent years, hydroquinone (HQ), and its clathrates, have become materials of great interest, due to their potential applications in the * Michael T. Ruggiero [email protected] * Daniel M. Mittleman [email protected]
1
School of Engineering, Brown University, 184 Hope St, Providence, RI 02912, USA
2
Department of Chemistry, University of Vermont, 82 University Place, Burlington, VT 05405, USA
Journal of Infrared, Millimeter, and Terahertz Waves
energy-related sciences and industries. Hydroquinone is highly selective in the rate of forming clathrates, depending on the guest species, and thus may prove useful in various applications including gas separation, decarbonization, and global warming prevention [1, 2]. Hydroquinone is also a promising hydrogen-storage material [3–5], as well as a platform for studying quantum behaviors of trapped molecules [6, 7]. Various measurement approaches, such as X-ray diffraction, Raman spectroscopy, nuclear magnetic resonance spectroscopy, and neutron diffraction have been applied to understand the properties of HQ and its clathrates [1, 8–10]. Given the importance of weak and non-covalent intermolecular forces for the formation and properties of HQ clathrates, there is a need for experiments that can effectively probe these phenomena. Terahertz time-domain spectrosc
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