Structural phase transitions and Raman spectra of pyridinium perrhenate at high pressures

The crystal structure and vibrational spectra of deuterated pyridinium perrhenate (d5PyH)ReO4 (C5D5NHReO4) have been studied by means of X-ray diffraction and Raman spectroscopy at high pressures up to 15 and 9.5 GPa, respectively. Two phase transitions, from orthorhombic phase II with Cmc21 symmetry to phase I with Cmcm symmetry at P = 0.7 GPa and to new monoclinic phase IV with P21 symmetry at P = 5 GPa were observed. The internal vibrational modes frequencies, lattice parameters and unit cell volume as functions of pressure were obtained for different phases.