A simple procedure, with an available program, may readily be used to establish the three-dimensional relationships between a mother and daughter phase, where mother and daughter are either two different phases of a material or a reactant and product in a solid-state transformation. The only requirements are that (i) the process involves a crystal-to-crystal transformation, and (ii) the experiment must be carried out without changing the alignment of the mother crystal. Application of the method supports the inferred alignment of mother and daughter phases in the published structures of two polymorphs of 4-cyanopyridinium perchlorate monohydrate 1. A new low-temperature, monoclinic polymorph of ferrocenium tetrafluoroborate 2 is produced when the known orthorhombic polymorph is cooled from 173 to 120 K. The major features of the transformation include "conservative twinning" and a modulation of the structure along the crystallographic b direction upon cooling. Redetermination of the structure of the low-temperature polymorph of ferrocenium hexafluorophosphate 3 reveals that the phase change is accompanied by reproducible four-component twinning, providing a proper explanation for the previously reported, very high R-factor of 12.4%. Included tutorial information on the process will assist the reader in obtaining topotactic relationships, as well as in preparing figures and animations describing phase transitions or reactions.

A straightforward procedure is outlined for prediction of the complete three-dimensional coordinate set for a highly disordered phase. For a mother and daughter phase, where one of the pair has an “unsolvable” structure, one needs only to (a) establish the topotaxy using previously published techniques, (b) obtain the topotactic transformation matrix, φ, between the ordered and highly disordered phase, and (c) apply the transpose of φ–1 to obtain a trial set of coordinates for refinement using the reflection data set of the highly disordered phase. For the inclusion compound [Fe­(η-C5H5)]­·3­(NH2)2CS (1), which contains highly disordered ferrocene molecules above 160 K (polymorph 1_I), we found a more ordered structure at low temperature. At 135 K (polymorph 1_II), two ferrocene moieties are present in the thiourea channel in an approximately 1:1 ratio. One is nearly orthogonal (87.0°) to the channel axis, while the other is tipped 16.2° from that direction. Using steps (a–c) outlined above, a trial structure may be obtained for 1_I, and refinement leads to R 1 = 4.25%. The structure of 1_1, containing 12-fold disordered ferrocene molecules, is similar to that found at temperatures below the phase transition, with a greater amount of the orthogonal orientation (55:45 vs 51:48), consistent with, but lower than, amounts found using solid-state NMR techniques. The low temperature polymorph is a trill, with an approximately 3:1:1 ratio of twin components. The exact alignment of the mother phase and the three daughters has been established using the methods of topotactic analysis described previously.

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