Growth-Induced Effects of Polarity in Molecular Crystals: Comparison of Schottky-and Markov-Type Models with Monte Carlo Simulations

摘要

Crowth-induced 180deg C orientational defect formation in centric and acentric organic crystals built up by polar molecules is described by Schottky-and Markov-type theories and compared to the Monte Carlo simulations. Good numerical agreement between the three approaches is obtained for a defect level lower than a few percent. From the temperature dependence of defect probability, we predict that in real materials crystallizing in a centric group, a defect level typically of a few percent or more may develop if growth occurs at elevated temperature. Observed second harmonic generation effects in organic crystal structures refined to an wR(F~2) value of a few percent are compatible with an orientational defect level on the same order. Concerning the mechanism of defect formation, healing of primary defects is likely for a realistic range of intermolecular couplings in various materials. In the case of growth upon a polar seed, the system is developing a stable configuration of polarity along one of the directions of the polar axis, whereas for the opposite direction, switching of the majority of dipoles is predicted.