Microscopic theory of orientational disorder and lattice instability in solid ${\mathrm{C}}_{70}$

We have developed a microscopic theory which describes the orientational dynamics of ${\mathrm{C}}_{70}$ molecules and its coupling to lattice displacements in the face-centered-cubic phase of ${\mathrm{C}}_{70}$ fullerite. The single-molecule orientational density distribution in the disordered phase is calculated. The ferroelastic transition to the rhombohedral phase is investigated. The discontinuity of the orientational order parameter at the phase transition is calculated. It is found that the transition leads to a stretching of the primitive unit cell along a [111] cubic direction. A softening of the elastic constant $c_{44}$ at the transition is predicted.