Coupling of orientational and translational modes in solid C₆₀and C₇₀

Abstract

The orientational phase transitions in solid C60 and C70 are accompanied by-quite different anomalies in the crystalline strains. In solid C~O the phase transition Fm3m + Pa3 is primarily an orientational effect (antiferro-rotational), which is driven by the condensation of orientational modes belonging to X: irreducible representation (irreps) of Fmjm. These modes are the primary order parameters (oops) and their number is equal to the number of i m p s of Tzg and T I , symmetry within the manifolds under consideration. Taking into account irreps up to the manifold I= 12, we have studied the rotation-rotation-translation (RRT) coupling between the oops and the lattice displacements. We have investigated the resulting lattice contraction and the change of the elastic constant c1I at the phase transition. In solid C70 (fcc-phase) we investigate the bilinear coupling of orientational fluctuations of Tzg symmetry to transverse acoustic lattice displacements. This coupling is the driving mechanism for the ferroelastic phase transition Fm3m + Rjm. Finally we investigate the transition from the rhombohedra1 phase to a low temperature monoclinic phase. This transition in antiferro-rotational.