High pressure crystal phases of benzene probed by infrared spectroscopy

In this work we present an infrared study of the high pressure phases of be nzene crystal up to 25 GPa. The far infrared portion of the spectrum has be en studied for the first time above 4 GPa. From the pressure behavior of th e frequencies and the integrated peak absorptions we are able to confirm, b eside the transition between phases II-III at 4.8 GPa, the transition betwe en 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 wit h a crystal structure having two molecules per cell and the inversion cente r 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 t he crystal modes. Particularly important are the changes of the doubly dege nerate nu (18) bending mode, which could be strongly involved in the irreve rsible transformation occurring at higher pressure. (C) 2001 American Insti tute of Physics.