Phonon spectrum of a naphthalene crystal at a high pressure: Influence of shortened distances on the lattice and intramolecular vibrations

The Raman spectra of a naphthalene crystal have been measured at room temperature in the pressure range up to 20 GPa. The pressure shift and Grüneisen parameters for intermolecular and intramolecular phonons have been determined. The maximum rate of the pressure shift for intermolecular phonons is 44 cm$^{-1}$/GPa, and the rate of the pressure shift for intramolecular phonons lies in the range from 1 to 11 cm$^{-1}$/GPa for different modes. The pressure dependence of the phonon frequencies for direct and inverse pressure variations has a hysteresis in the pressure range from 2.5 to 16.5 GPa. It has been shown that the linear dependence of the intermolecular phonon frequency on the crystal density has a peculiarity, which indicates a possible phase transition at a pressure of 3.5 GPa. The pressure dependence of intramolecular phonons related to the stretching vibrations of hydrogen atoms exhibits features that are characteristic of intermolecular phonons, which is associated with the influence of shortened distances between the hydrogen atoms of the neighboring molecules on the intermolecular interaction potential.