Structures and phase transitions in C60 and C70 fullerites

The room-temperature face-centered cubic (FCC-a0) phase as well as the low-temperature simple cubic (SC) phase of C60 are studied by electron microscopy and electron diffraction. The micro-structure of the room-temperature FCC-a0 phase is very similar to that of a low stacking fault energy FCC alloy; micro twins and stacking faults on the {111} planes are the main defects. High-resolution observations of these defects are presented here. In some parts the high-resolution images suggest a reconstructed lattice at the surface, possibly due to the presence of oxygen. The phase transition FCC-a0 ⇒ simple cubic (SC) at 255 K is confirmed and the observed reflections in the SC phase are only compatible with the space group Pa3¯. A second phase transition SC ⇒ FCC-2a0 is reported. It occurs presumably at a slightly lower temperature. It is suggested that in the SC phase the molecules still have some rotational degree of freedom about their respective <111> rotation axis. In the FCC-2a0 phase the rotation angle is assumed to be frozen in and to alternate between +φ and -φ along the <100> directions. C70 can grow either hexagonally close-packed (HCP) or cubic close-packed (FCC). On cooling the HCP crystals undergo a phase transformation whereby the c/a ratio increases from 1.64 to 1.82. At lower temperatures the C70 molecules orient themselves inside the close-packed planes to induce a monoclinic C-centered superstructure. Above room temperature a shear transformation from ABAB... to ABCABC... occurs. Due to the ellipsoidal shape of the molecule, however, the structure is only rhombohedral with α = 88°-89°. Only at higher temperatures does the structure gradually become FCC.