Crystal structure and morphology of the bright orange γ-phase of Pigment Red 53:2 from XRPD, DFT+D and TEM

Abstract Pigment Red 53:2 (P.R.53:2), the calcium lake of P.R.53, in its γ-phase is a bright orange pigment suitable for printing applications and for the colouration of seeds. The γ-phase forms needle-like, long thin crystals, which are often bent, as seen by transmission electron microscopy (TEM). Its crystal structure was determined from synchrotron X-ray powder diffraction data supported by lattice-energy minimisations with dispersion-corrected density functional theory (DFT+D). The γ-phase is a dihydrate with the chemical composition Ca(C17H12ClN2O4S)2 · 2 H2O. The calcium ion is coordinated by seven oxygen atoms, which come from the sulfonate groups of four anions, one carbonyl group and two water molecules. The structure contains columns which are built by the Ca2+ ions, the water molecules, and the pigment anions, with their sulfonate and carbonyl groups directed inward, and their non-polar phenyl and naphthalene groups pointing outwards. All columns are parallel. Neighbouring columns are connected by van der Waals forces only. This explains the observed bend, hair-like morphology of the crystals. Remark: This manuscript must not be published online in its present state without written permission of the authors.