Pressure effects on the crystal structure of the cubic metallofullerene salt [Li@C60][PF6] to 12 GPa

The pressure-dependent structural properties of the metallofullerene salt, Li+@C60][PF6]−, which has been shown before to adopt a primitive cubic structure at ambient conditions, have been studied by synchrotron X-ray powder diffraction at ambient temperature and at 7 K up to ~12 GPa. We find no evidence for a phase transition across the accessed pressure range at either temperature with the structure always remaining strictly primitive cubic (space group Pa3̅). The extracted bulk moduli, K0 of 17.51(13) GPa and 16.6(2) GPa at 7 K and ambient temperature, respectively are comparable to those of other cubic close-packed fullerene solids implying little influence of the interaction between the endohedral Li+ and the interstitial [PF6]− unit. Rietveld analysis of the diffraction data shows that the adopted primitive cubic structure incorporates orientationally ordered [Li+@C60] units – they are rotated anticlockwise by ~101° about the three-fold [111] rotation axis. This finding together with the lack of observation of a monomer→polymer phase transition upon pressurization contrast sharply with the structural behaviour of isostructural pristine C60 and its intercalated salts, (deely quenched) CsC60 and Na2CsC60, which both incorporate orientationally disordered C60 units and have a tendency to easily polymerize via intermolecular C−C bond formation. Such differences can be understood in terms of the expanded nature of the salt and the steric interactions introduced by the large [PF6]− unit residing in the octahedral interstices.