Elastic and anharmonic properties of pyridinium tetrafluoroborate([C5NH6]+[BF4]- ≡ PyBF4):: light scattering study of the phase transitions at various pressures

The vibrational properties of pyridinium tetrafluoroborate crystals were studied by means of Brillouin scattering at atmospheric pressure, Raman scattering and neutron diffraction under high pressure. It is shown that the elastic properties strongly reflect the molecular anisotropy of the pyridinium ions. The compressibility obtained from the Brillouin scattering study is in reasonable agreement with the results from neutron crystallography under pressure. Brillouin spectra reveal a small quasielastic component at room temperature, which is described by a coupled-susceptibilities formalism applied to the longitudinal mode and a relaxation mode. The mode Gruneisen parameters for all three phases that are found are determined from the pressure and temperature dependence of the Raman spectra and from neutron experiments.