Computational investigation of surface structural relaxation in crystalline urea

Abstract

The effects of surface structural relaxation on the energetic and morphological properties of crystalline urea have been investigated. Specifically, the {001}, {110}, {200}, {101}, {111} and {[graphic ommitted]} surfaces have been considered. Upon relaxation of the cleaved surfaces, significant perturbations in the positions and internal geometries of the molecules (compared with the corresponding positions in the bulk crystal structure) are observed. Interestingly, the {[graphic ommitted]} surface has a lower surface energy than the {111} surface, and thus a polar morphology is predicted. The predicted crystal shape is compared with that observed experimentally (for crystals grown by deposition from the vapour phase), and reasons for discrepancies between the calculated and experimental morphologies are discussed.