Definition of Topotaxy

THE term ‘topotaxy’ was defined by Lotgering and Gorter 1 to include “all chemical solid state reactions that lead to a material with crystal orientations which are correlated with crystal orientations in the initial product”. Mackay 2 described several reactions occurring in the hydrated iron oxides as topotactical and extended the definition to include structural transformations. He stated the necessary conditions for topotaxy to be: (1) a three-dimensional accord between the reactant and product; (2) the majority of atomic positions remaining fixed. In a review of topotaxy, Bernal 3 defined the term so as to include transformations in which all or some of the axes of symmetry of the original crystal are retained in the product. Taylor et al. 4 recognized the problem of the definition of topotaxy and took a position intermediate between that of Lotgering and Mackay. They refer to topotaxy as the conversion of a single crystal into one or more products which have a definite crystallographic orientation with respect to the original crystal, with the added requirements that the conversion occur throughout the entire crystal and that there be three-dimensional accord between the initial and final structures.