Isosymmetric temperature-triggered structural phase transition of dabcodiium chlorochromate chloride

Dabcodiium chlorochromate chloride was synthesized and separated as crystals. DSC measurement shows that this compound undergoes a reversible phase transition at about 185 K with a heat hysteresis of 5 K. Step-like dielectric anomaly observed at 187 K further confirms the phase transition. The crystal structures determined at 173 and 296 K are all monoclinic in P21/m, showing that the phase transition is isosymmetric. The transition is realized by tripling the a-axis without changing the b, c-axes, and β angle. A 60° rotation of one of every three CrO3 groups around the Cr-O bond is probably the driving force of the transition. Hydrogen bonding effect on the transition was shown to be negligible.