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Abstract:
The phase transition in C60 and (related) lattice defects are studied by low-temperature electron microscopy and electron diffraction. The microstructure of the room-temperature face-centered-cubic (fcc-a0) phase is very similar to that of a low stacking fault energy fee alloy; micro twins and stacking faults on the {111} planes are the main defects. The phase transition fcc-a0 ⇒ simple cubic (sc) at 249 K, is confirmed by single-crystal diffraction and the space group of the sc phase was unambiguously determined from the systematic extinctions as Pa3. Moreover a second phase transition, sc ⇒ fcc-2a0, is discovered. It occurs at a slightly lower temperature. It is suggested that in the sc phase the molecules still have a rotational degree of freedom about their respective 〈111〉 rotation axis, of which the orientation pattern is already fixed by the Pa3 space group. In the fcc-2a0 phase the rotation angle is found to be frozen in and to alternate between +φ and -φ along the 〈100〉 directions. The domain structure of the sc phase consists of eight variants (four translation variants for each of the two orientation variants) present in rather ill-defined regions. As a consequence of the ease of rotation of the molecules, no sharp interfaces are formed between different orientation domains.
