Mean field approach to orientational transitions in charge–transfer molecular crystals

Abstract

The mean field approach to orientational transitions in two-component molecular crystals has been formulated and illustrated by numerical calculations for two model systems: complexes of tetracyanobenzene (TCNB) with naphthalene (N–TCNB) and anthracene (A–TCNB). The study had addressed the characterization of the orientational disorder and mechanism of phase transitions in the crystals by calculating single particle and coupled rotational susceptibilities. It is concluded that the orientational instability in A–TCNB crystal is of the displacive type and a soft librational phonon mode at the M point of the Brillouin zone is predicted. In the N–TCNB crystal, an order–disorder type of the instability is predicted. For both systems the orientational phase transitions are found to be predominantly two dimensional.