Pressure–Temperature Phase Diagrams and Transition Mechanisms of Hybrid Organic–Inorganic NH···N Bonded Ferroelectrics

Abstract

The ferroelectric–paraelectric transitions of 1,4-diazabicyclo[2.2.2]­octane (dabco) tetrafluoroborate (dabcoHBF4) and perchlorate (dabcoHClO4), between their phases II (orthorhombic space group Pm21 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 Å and become disordered, but the protons remain ordered in the NH+···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. DabcoHBF4 has been also investigated at high pressure and its new phases have been revealed. The high pressure phases IV of dabcoHBF4 and III of dabcoHClO4 are isostructural. High-pressure in situ crystallizations in a diamond-anvil cell yield also hydrate dabcoHBF4·H2O of an unprecedented structure, where the crystallization water molecules are OH···O bonded into zigzag chains parallel to the NH+···N bonded cations.

Comment on “Improper molecular ferroelectrics with simultaneous ultrahigh pyroelectricity and figures of merit” by Li et al.

“…Despite the wrong space group and lattice parameters, the structural model presented by Li et al . in their figure 1 (D and F) is consistent with the correct larger unit cell and symmetry determined in our single-crystal experiments ( 5 ). The symmetry relation between the ferroelectric and paraelectric phases is of primary importance.…”

“…They incorrectly describe the symmetry of paraelectric phases of dabcoHClO 4 and dabcoHBF 4 as space group P 4/ mmm (No. 123) with the unit cell containing one ionic pair ( Z = 1), while, as shown in our previous study ( 5 ), the true space group is P 4/ nmm (No. 129) and the unit cell is two times larger ( Z = 2).…”

“…The structural and symmetry information is essential for understanding the mechanism underlying the ferroelectric properties and phase transitions in dabco monosalts. Therefore, we have performed more than 50 single-crystal x-ray and neutron diffraction experiments ( 3 , 5 – 7 ), as a function of temperature and pressure. The single-crystal measurements circumvent the overlapping of reflections inherent to powder x-ray diffraction (PXRD), hampering the precise structural determinations and location of the disordered atoms and protons.…”

“…( D ) Spontaneous polarization measured for single-crystal dabcoHBF 4 across the ferroelectric-paraelectric and ferroelectric-ferroelectric phase transitions according to Budzianowski et al . ( 7 ) (right axis) and the θ angles (left axis) determined for dabcoHClO 4 and dabcoHBF 4 from our structural studies ( 3 , 5 – 7 ).…”

Response to Comment on “Improper molecular ferroelectrics with simultaneous ultrahigh pyroelectricity and figures of merit”

“…Szafrański and Katrusiak stated that the space group of the paraelectric structure of [Hdabco]ClO 4 and [Hdabco]BF 4 is P 4/ mmm ( 1 – 4 ). In 2018, they overturned their previous results and suggested that the space group is P 4/ nmm ( 5 ) according to neutron-diffraction measurements. While the subtle nature of P 4/ mmm and P 4/ nmm structures could be appreciated from the marked drop of the intensity of the reflections with the sum of Bragg indices h + k odd, Szafrański and Katrusiak misused the reflection (034) in their neutron diffraction measurements [figure 4 of ( 5 )], which unfortunately did not obey the Bragg indices h + k odd ( 6 ).…”