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

The vibrational properties of pyridinium tetrafluoroborate crystals were st udied by means of Brillouin scattering at atmospheric pressure, Raman scatt ering and neutron diffraction under high pressure. It is shown that the elastic properties strongly reflect the molecular anis otropy of the pyridinium ions. The compressibility obtained from the Brillo uin scattering study is in reasonable agreement with the results from neutr on crystallography under pressure. Brillouin spectra reveal a small quasiel astic component at room temperature, which is described by a coupled-suscep tibilities formalism applied to the longitudinal mode and a relaxation mode . The mode Gruneisen parameters for all three phases that are found are det ermined from the pressure and temperature dependence of the Raman spectra a nd from neutron experiments.