Structural Anharmonicity Explains Continuous Frequency Shifts of Intramolecular Ring Vibrations in a Hydrogen-Bonded Antiferroelectric Crystal

Abstract

Rational principles from which one can design the energy storage capability of antiferroelectric crystalline materials composed of hydrogen-bonded molecular constituents remain deficient. In particular, there remains gaps in our understanding of the fundamental physical mechanisms by which microscopic electrostatics in this class of materials control macroscopic properties central to potential energy storage technologies. While molecular vibrations have been applied as probes of microscopic electrostatics in other areas of materials physical chemistry, their ability to report the electrostatics in antiferroelectrics remains an open question. In this study, we use temperature-dependent Raman spectroscopy to investigate the fundamental physical mechanism by which aromatic ring distortional vibrational peaks shift in the antiferroelectric phase of crystalline 2-trifluoromethylbenzimidazole (TFMBI). With density functional theory (DFT) calculations and a theoretical model of anharmonic contributions to vibrational frequencies, we propose the quartic anharmonic contribution to the interatomic potential energy of TFMBI explains the continuous vibrational peak shifts we observe in the temperature-dependent Raman spectra of this material. Specifically, we find the shifts likely result from thermally driven changes to the average occupation of other, lower frequency intramolecular vibrations interacting with the Raman-active ring distortion vibrations of crystalline TFMBI. To support this conclusion, we use explicit fits of our measured peak shifts to an equation for the fundamental vibrational transition we develop from a theoretical model of material anharmonicity and the harmonic frequencies found from DFT calculations. In addition to highlighting the importance of anharmonic contributions to the structural dynamics of molecular crystals, this analysis also allows us to assess the ability of DFT calculations to predict the harmonic frequency of the aromatic ring vibrations of TFMBI correctly. Furthermore, we characterize the correlation between the structure of a low-temperature monoclinic phase of TFMBI unreported previously and discontinuous shifts in the positions of three peaks in the Raman spectrum of TFMBI. This characterization indicates the quartic anharmonic coupling does not depend on the hydrogen-bonding structure of the material. Our results illustrate the care researchers must take to appropriately interpret the physical mechanism explaining vibrational peak shifts in hydrogen-bonded antiferroelectric materials.