Two-stage rotational disordering of a molecular crystal surface: C-60 - art. no. 085503

We propose a two-stage mechanism for the rotational surface disordering pha se transition of a molecular crystal, as realized in C-60 fullerite. Our st udy, based on Monte Carlo simulations, uncovers the existence of a new inte rmediate regime, between a low-temperature ordered (2 x 2) state, and a hig h-temperature (1 x 1) disordered phase. In the intermediate regime there is partial disorder, strongest for a subset of particularly frustrated surfac e molecules. These concepts and calculations provide a coherent understandi ng of experimental observations, with possible extension to other molecular crystal surfaces.