Templates and Twinning in Topotaxy

The oriented growth of a product phase within a reactant crystal (topotaxy) may be directionally unique if directional point‐group symmetry elements of the reactant crystal structure are preserved in the aligned crystal structure of the product. When the specific topotaxy results in an angular divergence between these elements, multiple orientations of the product phase may grow within the pseudomorph (conservative twinning). Irrespective of the specific topotaxy, the total product phase appears to orient so as to conserve the point‐group symmetry elements of the initial lattice. Several examples of conservative twinning in transformations of 2‐iododibenzoyl peroxides and the closely related benziodoxole structures are presented together with matrices describing the specific topotaxies. In most cases, several oriented products are simultaneously and consecutively formed and it is difficult a posteriori to define their individual roles in the transformation. Conservative twinning and sometimes its conspicuous absence provide important clues for the identification of that lattice which controls the alignment of a particular product phase.