Structure and Lattice Dynamics of C70 Single Crystals

Abstract

Single crystals of C70 are grown from the vapour phase using chro-matographically purified C70 powder as starting material. Most of the crystals grown have the fcc habit but a minority fraction shows a perfect hep morphology. These hep grown crystals are also microscopically hep as evidenced from X-ray and electron diffraction studies. On lowering the temperature the ideal hcp crystals undergo two phase transitions due to a sequential freezing out of the rotations of the molecules around their two different axes, resulting in an orientationally ordered low-temperature monoclinic structure. Raman measurements on C70 crystals in this low-temperature structure reveal a large number of Raman active inter-molecular librational and vibrational modes, consistent with the selection rules for monoclinic P21/m-C70. Harmonic lattice dynamics calculations on the low temperature structure of solid C70 have been performed and a lattice structure in agreement with the experiments is found. From these calculations additional information on the orientational ordering is obtained and the observed Raman active lattice modes can be quantitatively interpreted.KeywordsElectron Diffraction StudySingle Photon IonizationElectron Diffraction ExperimentSingle AngleCalculated Raman SpectrumThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.