The organic compound 2,2,3,3,4,4-hexafluoropentane-1,5-diol
(HFPD) was found to undergo a temperature-dependent phase transition between
two closely related triclinic polymorphs. Each of the two phases is twinned
with either two or four individuals depending on the temperature cycling
history. The molecules form sheets that slide with respect to others during
the phase transition. The phase transition is triggered by the natural contraction
of the unit cell upon cooling coupled with several unfavorable C-H···F and
F···F non-bonded contacts. Link to Abstract.
The superposition of (012) layers of forms
(Ib) and (II), is presented to illustrate sliding of the layers during the
enantiotropic phase transition of HFPD. A composite image is shown, containing
(012) layers for forms (Ib) and (II) above the basal layer. Molecules of
form (Ib) are drawn with dashed bonds and molecules of form (II) are drawn
with solid bonds. Each layer slides 1.83 (5) Å along the [01-3] axis in the (012) plane, based on the translation of respective pairs of O atoms in forms (Ib) and (II).