Coupling of rotational modes in a phase transition of a charge-transfer crystal

Abstract

ESR spectra of triplet excitons in the charge-transfer crystal anthracene- d 10− s-tetracyanobenzene (TCNB) have been used to investigate changes in the molecular orientation. The data yield an order parameter describing the orientational phase transition at 196 K. This transition is described using Landau theory and is shown to be of first order. The change of a second order parameter has also been detected. A coupling relation between the two order parameters and the influence of this coupling on the nature of the phase transition are discussed.