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1,4-Diazabicyclo[2.2.2]octane perchlorate,[C6H13N2]+ClO4–, dabcoHClO4, the first NH+···Nhydrogen-bonded ferroelectric, exhibits at least ten unsolvated polymorphs,depending on temperature and pressure. When crystallized from themethanol solution above 2 GPa, dabcoHClO4crystallizeswith two molecules of methanol to form solvate dabcoHClO4·2CH3OH. This solvate structure has been determinedby single-crystal X-ray diffraction up to 3 GPa. High pressure transformsthe H-bonding pattern and the hierarchy of different cohesion forces.The solvate formation is additionally favored by its molecular volumelower than the sum of the molar equivalents of unsolvated components. [ABSTRACT FROM AUTHOR]

Exceptionallyrich diagram of ten phases, I–X, has beenrevealed for ferroelectric 1,4-diazabicyclo[2.2.2]octane perchlorate,[C6H13N2]+ClO4–, dabcoHClO4. Pressure- and temperature-induced transformations of thisNH+···N hydrogen bonded structure have beencharacterized by single-crystal X-ray diffraction, calorimetric, anddielectric measurements. At high pressure the single crystals werein situ grown in a diamond-anvil cell at isochoric conditions. Thereare nine phases of dabcoHClO4between 400 and 11 K at ambientpressure, whereas phase X is formed at 1.5 GPa and 296 K. Tetragonalphase I space group P4/mmmtransformsto orthorhombic phases II (Pm21n), III (Pc21n), IV (Pc2a), and V (orthorhombic-modulated)on decreasing temperature. Phase II is stable up to 0.22 GPa, at 0.6GPa phase III is formed, and above 1.50 GPa a new phase X (Pc21a) has been obtained. Atnormal conditions dabcoHClO4is isostructural with theferroelectric crystal of dabcoHBF4; however, their phasediagrams are very different. Above 1.8 GPa dabcoHClO4cocrystallizeswith solvate molecules. [ABSTRACT FROM AUTHOR]