Abstract
The thermal expansion of single- and polycrystalline C70 is studied between 5 and 315K using an ultrahigh-resolution capacitance dilatometer. We observe a first-order phase transition in single crystalline C70 at ≈240K and a transition at ≈280K in polycrystalline C70. X-ray diffraction measurements at 300K show that the C70 single crystal has fcc symmetry, whereas the polycrystalline sample is multiphase (fcc and hcp). The results are compared to thermal expansion measurements of solid C60.
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We define f as the volume of the molecule, as given by the eilipsoid determined by the C nuclei, divided by 1/4 of the fcc unit cell volume at 300K. We used r=3.55Å and [r a=3.65Å, r b=4.25Å, see W. Andreoni et al. Chem. Phys. Lett. 189, 241 (1992)] as the radii of the C60 and C70 molecules, respectively, and the lattice parameters were taken from [3, 5, 7]
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