Orientational correlations in the high-temperature phase of solid C60

The diffuse scattering in the high-temperature phase of solid C₆₀ forms a very complicated pattern. It is analyzed from microscopic interactions within a mean-field theory. Different local orientational configurations of the C₆₀ molecules are found.     

Free energy and orientational phase transition in solid C60

On the basis of a microscopic model, the free energy of the orientationally disordered fcc crystalline phase of C₆₀-fullerite is derived up to fourth order in the rotational density fluctuations. The order parameter variables are symmetry adapted rotator functions which belong to thel=6 andl=10 manifolds. They are basis functions ofT _2g representations of the cubic group. The phase transition to thePa3 structure is discussed, and is found to be of first order. The orientationally ordered phase is described in terms of condensed rotator functions.     

Orientational states of C60 molecule in crystals

Local symmetry of orientational states of the C₆₀ molecule in crystals has been investigated. It was shown that the various orientational phase transitions in different crystals are related to different orientational orbits. The model of orientational phase transitions based on a sequence of orientational states with different symmetry properties has been suggested. We have found that both the local symmetry of C₆₀ molecule and the symmetry of its internal vibrations become higher after a reduction of crystal spatial symmetry at the phase transition. This effect is fairly common and can be observed in the orientational order-disorder phase transitions with wave vectors at the Brillouin zone boundary. Feasible manifestations of the predicted effect in various experiments are discussed. Zh. éksp. Teor. Fiz. 113, 1081–1093 (March 1998)     

Orientational correlations and order in A3C60

A tight-binding method which has been previously applied to study the effect of uncorrelated orientational disorder on conduction-band properties is extended here to the case of systems with long-range order and/or short-range correlations. The density of states and conductivity are not highly sensitive to the specific short-range correlations, so long as the system is not too close to being fully ordered. Hence the strong effects of disorder found previously appear to be robust and should play an important role in the interpretation of normal-state properties of A₃C₆₀.