9,10 Phenanthrenequinone, not your average structure

9,10-phenanthrenequinone, C₁₄H₈O₂, is both polymorphic and polytypic. The β form can have different symmetries arising from different stackings of ordered layers perpendicular to c_p*. These structure options are best understood using occupancy wave modulations of a 1:1 disordered parent structure in space group P2₁/c. The disorder is resolved by having ordered layers of C1 symetry in a cell with a = 2a_p, b = 2b_p. Ordering the stacking sequence allows two orientations of a Z = 8, Cc, c = c_p structure and two orientations of a Z = 16, F1, c = 2c_p structure. It was found that or some crystals it is necessary to model the reflection data assuming uncorrelated mosaic blocks of different space group symmetry as well as orientation. Reliable geometry for both the Cc and F1 components were then obtained. Likewise, the α form can be described as arising from a 1:1 disordered parent structure in space group Pnmn where the disorder is resolved by having ordered layers of P2/c symmetry perpendicular to a_p* in a cell with b = b_p, c = 2c_p. Ordering the stacking sequence allows two orientations of a Z = 4, P2₁cn structure with a = a_p and two orientations of a Z = 8, B2/c structure with a = 2a_p (a non standard setting of P2/c).