Structural Anharmonicity Provides a New Prospective of Vibrational Frequency Shifts in Ferroelectric and Antiferroelectric Materials

There remain fundamental gaps in understanding how the microscopic electrostatics of hydrogen-bonded antiferroelectric molecular crystalline materials, such as 2-trifluoromethylbenzimidazole (TFMBI), control the macroscopic properties essential for their application in light generation, photovoltaic conversion of solar energy and electrostatic energy storage technologies. To determine if molecular vibrations can assess the microscopic electrostatics we conducted temperature-dependent Raman measurements. We find vibrational peaks shift to higher frequencies across different regions of the spectra. We first concluded the vibrational Stark effect is not an explanation. With DFT and a theoretical model of anharmonic contributions to vibrational frequencies, we assessed whether structural anharmonicity could explain the peak shifts we observed. We conclude the peak shifts likely results from thermally driven changes to the average occupation of other lower frequency intermolecular vibrations interacting with the Raman-active ring distortion vibrations of TFMBI. Furthermore, we are actively investigating methyl-benzimidazole (MBI), a ferroelectric material, where we will decipher if vibrational probes report the microscopic electrostatic drivers of the macroscopic properties.