First Observation of the FCC to Trigonal/Rhombohedral Transition of Pure Dimerized C-60 Under High Pressure

Similarly to preformed polymeric phases such as 1D-Pmnn [O] or 2D-Immm [T], and in contrast to monomeric FCC C-60, dimeric C-60 [D or R'] is expected to yield new polymerized phases of higher dimensions when subjected to high pressure. We have carried out an in-situ high-pressure powder diffraction experiment in a diamond anvil cell using MoK alpha laboratory X-rays, on a pure dimeric sample previously carefully characterized at ambient pressure using synchrotron X-ray HR XRPD and Raman spectroscopy. A transition is indeed observed from 1.2 to 8.6 GPa. A robust analysis shows that the volume per C-60 unit decreases from 689 angstrom(3) (0 GPa) to 550 angstrom(3) (8.6 GPa), resulting in a 20% effect. The new phase can no longer be described by a cubic (FCC) unit cell, whereas it can now be indexed and LeBail-refined using a trigonal/rhombohedral unit cell predicted by Dzyabchenko et al. in 1999.