Experimental Establishment of Mother–Daughter Orientation Relationships and Twinning Effects in Phase Transitions: A Great Legacy from Jack Gougoutas and Peggy Etter

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.