Orientational Disorder and Order in C₆₀—Fullerite and in MC₆₀—Alkali Metal Fullerides

Abstract

We start with a review of the formalism of symmetry adapted functions (SAF) for the description of orientation dependent properties of solids. For nonlinear molecules SAF’s are rotator functions which take into account the symmetry of the molecule and the symmetry of the crystalline site. We apply these concepts to the description of the phase transition from the orientationally disordered phase (crystal structure $Fm\bar 3m$) to the ordered phase (crystal structure $Pa\bar 3$) in solid C₆₀. Due to the unusually high symmetry (I _h ) of the molecule, the first order phase transition is characterized by the simultaneous condensation of a multitude of order parameter components belonging to the irreducible representation $X_5^ + (\hat \tau ^{(9)} )$ at the X point of the Brillouin zone. Theoretical results are compared with recent neutron and X-ray diffraction experiments.