Growth-induced effects of polarity in molecular crystals: Comparison of Schottky- and Markov-type models with Monte Carlo simulations

Growth-induced 180 degrees orientational defect formation in centric and ac entric organic crystals built up by polar molecules is described by Schottk y- and Markov-type theories and compared to the Monte Carlo simulations. Go od numerical agreement between the three approaches is obtained for a defec t 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 gro wth occurs at elevated temperature. Observed second harmonic generation eff ects in organic crystal structures refined to an wR(F-2) value of a few per cent are compatible with an orientational defect level on the same order. C oncerning the mechanism of defect formation, healing of primary defects is likely for a realistic range of intermolecular couplings in various materia ls. In the case of growth upon a polar seed, the system is developing a sta ble 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.