Geometry and energetics of routes by which molecules are reoriented in a simple cubic phase of C60

Alternative routes of interconversion of two orientational molecular states ("double bond to pentagon" and "double bond to hexagon") coexisting in a simple cubic phase of C-60 crystals are considered. A systematic computer-aided search based on the CRYCOM code package for comparing crystalline structures resulted in two types of solutions corresponding to a simple (congruent) and mirror routes of interconversion of spatial configurations. The second-type solutions - routes, two of which involved rotation of molecules through smaller angles (Omega = 35 degrees and 38.1 degrees) than their earlier counterparts (Omega = 41.8 degrees and 60 degrees), were previously unknown. The possibility of both types of interconversion routes is in line with the effect of merohedral twinning. The method of atom-atomic potentials invoked to calculate the optimum trajectories of transitions between the potential :minima of the two states, passing through the appropriate stationary points, proved energetic unfavorableness of rotation of the molecules about the third-order axis compared to alternative routes. The mechanism of potential-barrier surmounting via relaxation of local nonequilibrium defect states formed by deceleration of independent rotations of the molecules is analyzed; the role of interfaces between domains in this mechanism is pointed out.