Average structures of the disordered β-phase of Pigment Red 170: a single-crystal X-ray diffraction study

The [beta]-phase of the industrially important Pigment Red 170 ([beta]-P.R. 170) has a structure with severe layer stacking disorder. The single-crystal X-ray diffraction pattern consists of a difficult-to-disentangle mix of Bragg diffraction superimposed on rods of diffuse scattering which impede the estimation of accurate Bragg intensities. Two average monoclinic structure models with the same unit-cell dimensions, but different extents of disorder in the layers and different space groups seem plausible, one with the non-conventional space group setting B21/g (No. 14, Z' = 2) and one in P21/a (No. 14, Z' = 4). Disordered molecules related by a translation of 0.158b are present in all layers of the B21/g model and in every second layer of the P21/a model. Layer-to-layer contacts are practically the same in both models. According to order-disorder theory, both models are valid superposition structures. Structure-factor calculations show that the pattern of strong and weak Bragg reflections is very similar for the two models. R factors indicate that the B21/g model is the most economic representation of the average structure. However, given the limitations in data processing, the P21/a model should not be discarded and further insight sought from a detailed analysis of the experimental diffuse scattering. The difficulties encountered in this analysis raise the question of whether or not the concept of an average structure is applicable in practice to [beta]-P.R. 170

Explanation of the stacking disorder in the β-phase of Pigment Red 170

The [beta]-phase of Pigment Red 170, C26H22N4O4, which is used industrially for the colouration of plastics, crystallizes in a layer structure with stacking disorder. The disorder is characterized by a lateral translational shift between the layers with a component ty of either +0.421 or -0.421. Order-disorder (OD) theory is used to derive the possible stacking sequences. Extensive lattice-energy minimizations were carried out on a large set of structural models with different stacking sequences, containing up to 2688 atoms. These calculations were used to determine the actual local structures and to derive the stacking probabilities. It is shown that local structures and energies depend not only on the arrangement of neighbouring layers, but also next-neighbouring layers. Large models with 100 layers were constructed according to the derived stacking probabilities. The diffraction patterns simulated from those models are in good agreement with the experimental single-crystal and powder diffraction patterns. Electron diffraction investigation on a nanocrystalline industrial sample revealed the same disorder. Hence the lattice-energy minimizations are able to explain the disorder and the diffuse scattering