New Polar Phases of 1,4-Diazabicyclo[2.2.2]octane Perchlorate, An NH+···N Hydrogen-Bonded Ferroelectric

Exceptionally rich diagram of ten phases, I–X, has been revealed for ferroelectric 1,4-diazabicyclo[2.2.2]octane perchlorate, [C6H13N2]+ClO4 –, dabcoHClO4. Pressure- and temperature-induced transformations of this NH+···N hydrogen bonded structure have been characterized by single-crystal X-ray diffraction, calorimetric, and dielectric measurements. At high pressure the single crystals were in situ grown in a diamond-anvil cell at isochoric conditions. There are nine phases of dabcoHClO4 between 400 and 11 K at ambient pressure, whereas phase X is formed at 1.5 GPa and 296 K. Tetragonal phase I space group P4/mmm transforms to orthorhombic phases II (Pm21 n), III (Pc21 n), IV (Pc2a), and V (orthorhombic-modulated) on decreasing temperature. Phase II is stable up to 0.22 GPa, at 0.6 GPa phase III is formed, and above 1.50 GPa a new phase X (Pc21 a) has been obtained. At normal conditions dabcoHClO4 is isostructural with the ferroelectric crystal of dabcoHBF4; however, their phase diagrams are very different. Above 1.8 GPa dabcoHClO4 cocrystallizes with solvate molecules.