J. Hulliger et al., Growth-induced effects of polarity in molecular crystals: Comparison of Schottky- and Markov-type models with Monte Carlo simulations, J PHYS CH B, 105(36), 2001, pp. 8504-8512
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.