Statics and dynamics of organic charge-transfer crystals: The orientational phase transition in A-TCNB

Abstract

Crystal potential energy calculations and lattice dynamical calculations are presented for the high-temperature and low-temperature phases of the charge-transfer complex crystal of anthracene with 1,2,4,5-tetracyanobenzene using the method of atom–atom potentials. For several different sets of the ‘‘exp-6’’ potential parameters the crystal energy was minimized with respect to the lattice constants and orientations and positions of the molecules in the unit cell. These calculations show good agreement with the experimental values. The equilibrium crystal structures were used to calculate the lattice dynamics in the harmonic approximation for both crystal modifications. In the phonon dispersion curves lattice modes were discovered that have imaginary frequencies which may be attributed to instabilities of the crystal. Application is made to the orientational phase transition occurring in this crystal. The results suggest that the high-temperature phase is dynamically disordered and that the phase transition is predominantly two-dimensional in the (ab) plane of the crystal and is driven by a librational phonon mode with soft mode behavior. In addition, instabilities are indicated in the stacking direction of molecules. The variance of the results with the parametrization of the potential energy function is studied.