From space group to space groupoid: the partial symmetry of low‐temperature E ‐vanillyl oxime

The phase transition of E ‐vanillyl oxime {1‐[( E )‐(hydroxyimino)methyl]‐4‐hydroxy‐3‐methoxybenzene, C 8 H 9 NO 3 } has been analysed by single‐crystal and powder X‐ray diffraction. The high‐temperature (HT) phase ( P 2 1 / a , Z ′ = 1) transforms into the low‐temperature (LT) phase (threefold superstructure, , Z ′ = 6) at ca  190 K. The point operations lost on cooling, { m [010] , 2 [010] }, are retained as twin operations and constitute the twin law. The screw rotations and glide reflections are retained in the LT phase as partial operations acting on a subset of Euclidean space . The full symmetry of the LT phase, including partial operations, is described by a disconnected space groupoid which is built of three connected components.