High pressure crystal phases of benzene probed by infrared spectroscopy

In this work we present an infrared study of the high pressure phases of benzene crystal up to 25 GPa. The far infrared portion of the spectrum has been studied for the first time above 4 GPa. From the pressure behavior of the frequencies and the integrated peak absorptions we are able to confirm, beside the transition between phases II-III at 4.8 GPa, the transition between phases III-III' at 11.2 GPa. The spectral pattern changes very smoothly among the different phases and the number of bands is always consistent with a crystal structure having two molecules per cell and the inversion center as a symmetry element. At 17.4 GPa a structural change is suggested from the slope decrease of the frequency evolution with pressure of almost all the crystal modes. Particularly important are the changes of the doubly degenerate nu (18) bending mode, which could be strongly involved in the irreversible transformation occurring at higher pressure. (C) 2001 American Institute of Physics.