Pressure-induced Fermi resonance between fundamental modes in p-hydroxybenzoic acid.

Fermi resonance (FR) is a representative anharmonic coupling phenomenon in molecular vibrations. High pressure provides an effective and controllable means to tune molecular structures, intermolecular interactions, and vibrational energies, thereby enabling access to anharmonic coupling regimes. Herein, pressure-dependent Raman spectroscopy was employed to investigate the vibrational behavior of p-hydroxybenzoic acid (PHBA) over the pressure range of 0.3-13.1 GPa. Upon compression, both internal and lattice vibrational modes shift toward higher wavenumbers and exhibited pronounced anomalies near 2.6 GPa, indicating a pressure-induced structural phase transition. Above the transition pressure, FR was observed between two fundamental vibrational modes located at 844 and 856 cm-1. These findings demonstrate that external pressure effectively modifies intermolecular interactions, vibrational characteristics, and anharmonic coupling, providing a tunable platform for exploring vibrational phenomena in hydrogen-bonded molecular crystals.