The twinning in 1-p-nitrobenzeneazo-2-naphthol (Para Red)

Crystals of Para Red have been studied by photographic X-ray methods. Cooling the crystals produced reversible splitting of the diffraction spots. This was traced to the use of crystals twinned by pseudo-merohedry, in association with anisotropic thermal contraction. At room temperature the twin obliquity was only 0° 22' and the diffraction spots of the two twin individuals were not resolved.