Pressure-Temperature Phase Diagrams and Transition Mechanisms of Hybrid Organic-Inorganic NH center dot center dot center dot N Bonded Ferroelectrics

摘要

The ferroelectric-paraelectric transitions of 1,4-diazabicyclo[2.2.2]-octane (dabco) tetrafluoroborate (dabcoHBF(4)) and perchlorate (dabcoHClO(4)), between their phases II (orthorhombic space group Pm2(1)n) and I (tetragonal space group P4/nmm) proceed at T-c = 377 and 378 K, respectively. On approaching phase I the anions are strongly shifted by about 0.8 angstrom and become disordered, but the protons remain ordered in the NH+center dot center dot center dot N bonds. In phase I, the disordered protons and anions approximate the tetragonal space group P4/mmm with a twice-smaller unit cell, but due to small displacements of the dabcoH(+) cations the crystal acquires the symmetry of tetragonal space group P4/nmm, with the unit cell compatible with that of phase II. The structures have been determined by single-crystal X-ray and neutron diffraction. DabcoHBF(4) has been also investigated at high pressure and its new phases have been revealed. The high pressure phases IV of dabcoHBF(4) and III of dabcoHClO(4) are isostructural. High-pressure in situ crystallizations in a diamond-anvil cell yield also hydrate dabcoHBF(4)center dot H O-2 of an unprecedented structure, where the crystallization water molecules are OH center dot center dot center dot O bonded into zigzag chains parallel to the NH+center dot center dot center dot N bonded cations.