On an intrinsic mechanism of surface defect formation producing polar, multidomain real-structures in molecular crystals

Abstract

Dipolar but achiral organic molecules may undergo orientational disorder in the bulk but particularly at the surface of a seed crystal. In cases where the activation energy for reversal of the dipoles in the bulk is much larger than that for reversal at the growing crystal faces, thermodynamic arguments predict that orientational disorder occurring at surface attachment sites can produce metastable but macroscopically large growth sectors, featuring different polar properties than the seed. If the process of nucleation produces a non-pyroelectric symmetry group, then orientational disorder at the surface may give rise to pyroelectric properties during subsequent growth. Similarly, for seeds belonging to a pyroelectric group, orientational disorder can reduce the initial pyroelectric effect. According to this new surface-related mechanism of intrinsic defect formation, acentric and achiral molecules are likely to form real-structures which show a pyroelectric effect, although from a crystallographic point of view, such crystals may be described topologically by e.g. a centric structure. These present views are applied to the space group P2₁/c, supplemented by comments on techniques available for an experimental investigation of the predicted real-structurs.