Abstract
9,10-phenanthrenequinone, C~14~H~8~O~2~, is both polymorphic and polytypic. The ฮฒ 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 __P__2~1~/c. The disorder is resolved by having ordered layers of __C__1ฬ
symmetry 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, __F__1ฬ
, c = 2c
~p~ structure. It was found that for 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 __F__1ฬ
components were then obtained. Likewise, the ฮฑ 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 __P__2/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, __P__2~1~
cn structure with a = a
~p~ and two orientations of a Z = 8, __B__2/c structure with a = 2a
~p~ (a non standard setting of __P__2/c).