High Pressure Structural Investigation of Benzoic Acid: Raman Spectroscopy and X-ray Diffraction

Abstract

The structural stability of benzoic acid (C 6 H 5 COOH, BA), a hydrogen-bonded molecular crystal, has been investigated by Raman spectroscopy and angle-dispersive X-ray diffraction (ADXRD) up to ∼18 GPa at room temperature. Under ambient conditions, benzoic acid molecules are arranged in two sets of parallel planes and held together by hydrogen bonding and van der Waals interactions. Small changes (e.g., emergence of new peaks, splitting of original peaks) can be observed in the Raman spectra at high pressures. However, no obvious changes can be observed in the X-ray diffraction measurements, which rules out any symmetry/structure changes within this pressure range. The pressure dependence of lattice parameters is presented, which shows monotonously decrease without any anomalies. The experimental isothermal pressure–volume data are well fitted by the third-order Birch–Murnaghan equation of state, yielding bulk modulus B 0 = 41.7(6) GPa and a first pressure derivative B 0 ′ = 4.5(4). Axial compressibility shows obvious anisotropy, the a axis is more compressible than b and c axes. Moreover, the near symmetrization limit of hydrogen bonds at high pressures is proposed from the first-principles calculations. Based on the Raman, XRD, and the first-principles calculations analysis, we propose that the high pressure structural stability of benzoic acid is associated with the special hydrogen-bonded dimer structure.