Lattice defects in quinacridone

Abstract

Various lattice defects in the αI-phase of quinacridone (C20H12N2O2) were simulated using lattice-energy minimizations. αI-Quinacridone forms a chain structure in P 1, Z = 1. The molecules are connected by hydrogen bonds along [010], by π-stacking along [100] and by weak van der Waals interactions along [001]. αI-Quinacridone is inherently nanocrystalline. Lattice defects were calculated in correspondingly large supercells with up to 4464 atoms, using a previously evaluated force field. Vacancies, vacancy aggregates and interstitial molecules are energetically very unfavourable. A misorientation of a single molecule (flip around [010] by 180°) causes an energy increase of 243.7 kJ mol−1. Various edge and screw dislocations were investigated. A screw dislocation along [010] causes an energy increase of ΔE = 38.0 kJ mol−1 per molecule, all other line dislocations are even worse. In contrast, the rotation of an entire chain around the chain axis [010] by 180° leads to only a very small energy increase (ΔE = 1.6 kJ mol−1) and the real crystals probably contain a high number of such defects. Various planar defects were calculated, including different stacking disorders and misfit-layer structures with two different types of layers having different lateral periodicities. Stacking faults along [001] with herringbone stacking instead of parallel stacking are energetically quite favourable (ΔE = 2.2 kJ mol−1); the same is true for domains with misoriented molecules in the [001] direction. As an example for a bulk defect, domains are calculated in which blocks of 4 × 4 chains are rotated by 180° around [010], which leads to an energy increase of only 1.1 kJ mol−1. Twinning by mirroring at the (001) plane is energetically favourable (ΔE = 0.9 kJ mol−1). This twinning was observed in an HRTEM image. It is probable that the crystallites also contain rotations of chains, layers or blocks around [010] by 180°, but these defects cause only a very slight modification of the molecular packing, which was not observable in the HRTEM image. The lattice defects in αI-quinacridone investigated here provide an insight into lattice defects, their energies and local structures. Similar lattice defects are expected to occur also in other similar organic chain structures.