Low-Temperature Phase Transition in a Biphenyl-Fullerene Single Crystal

We have investigated, by means of temperature-dependent vibrational spectroscopy, the properties of neutral single crystals Of C-60 intercalated with biphenyl. Raman scattering results from a biphenyl-C-60 single crystal in the temperature range 75-300 K are in support of the two reported phase transitions in the material, as determined by X-ray diffraction (at 147 and 212 K). Following the variations of the Raman features of the single components, biphenyl and C-60, it is Possible to infer the behavior of the two molecules at such temperatures. In particular, at least three of the main biphenyl mode energies and/or intensities plus the C-H Raman band show modifications correlating with the reported phase transition temperatures. From the variation of the relative intensity of two of the modes, a partial tentative qualitative picture of the biphenyl planarity as a function of temperature can be drawn. The behavior of most of the C-60 mode energies, widths, and intensities correlate with either of the transitions, but not necessarily both. Especially abrupt H, mode intensity modifications at similar to147 K seem to result from the influence of the corresponding phase transformation on C-60 electronic transitions, by changing the C-60 site symmetry.