Simple guanidinium salts revisited: room-temperature ferroelectricity in hydrogen-bonded supramolecular structures.

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.