Simple Guanidinium Salts Revisited: Room–Temperature Ferroelectricity in Hydrogen-Bonded Supramolecular Structures
journal contributionposted on 14.07.2011, 00:00 by Marek Szafrański
Dielectric, calorimetric, and X-ray diffraction methods have been employed to characterize the crystals of guanidinium tetrafluoroborate and guanidinium perchlorate, both built of two-dimensional honeycomb hydrogen-bonded sheets. The room-temperature ferroelectricity of these isosymmetric complexes (space group R3m) has been evidenced by the polarization switching in an external electric field and pyroelectric effect. The analysis of structural data as a function of temperature showed that the high values of spontaneous polarization of about 8.5 μC cm–2 originate mainly from the ionic displacements, while the exceptional thermally induced increase of polarization is related with the apparent weakening of the N–H···F/N–H···O hydrogen bonds at elevated temperatures. An excellent correlation between the donor–acceptor distance and the relative displacement of the ions in the crystal lattice along the polar direction has been found. The huge entropy change at the two-closely spaced high-temperature phase transitions in guanidinium perchlorate, together with the large crystal polarization, suggest a large electrocaloric effect, the property strongly desired for solid-state cooling applications.