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Temperature-dependent phase transition observed between polymorphic crystals

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-HF and FF 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).

 
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