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

Abstract

The pressure-dependent structural properties of the metallofullerene salt, Li+@C-60][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 similar 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 Pa (3) over bar). The extracted bulk moduli, K-0 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+@C-60] units - they are rotated anticlockwise by similar to 101 degrees 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 C-60 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.