The β-phase of the industrially important Pigment Red 170 (β-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 B2(1)/g (No. 14, Z' = 2) and one in P2(1)/a (No. 14, Z' = 4). Disordered molecules related by a translation of 0.158b are present in all layers of the B2(1)/g model and in every second layer of the P2(1)/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 B2(1)/g model is the most economic representation of the average structure. However, given the limitations in data processing, the P2(1)/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 β-P.R. 170.
Keywords: Pigment Red 170; average structure; diffuse scattering; disorder; layer stacking faults; synchrotron.