Proton Dynamics and Room-Temperature Ferroelectricity in Anilate Salts with a Proton Sponge

Ferroelectricity as well as characteristic proton-transfer dynamics are achieved by combining a 2,3,5,6-tetra(2′-pyridyl)pyrazine (TPPZ) molecule with anilic acids (H2xa). Dielectric measurements revealed phase transitions at Tc = 334 and 172 K for bromanilate (Hba−) and chloranilate (Hca−) salts, respectively. The room-temperature ferroelectricity of the (H2-TPPZ)(Hba)2 crystal is evidenced by the slow polarization reversal with modest pyroelectricity. In accord with the observed large deuteration effect, synchrotron X-ray diffraction studies disclosed proton dynamics in an intramolecular N···H+···N bond of the H2-TPPZ2+ dication and in an O−H···O− hydrogen-bonded cyclic dimer of the ortho-quinoid Hxa− anions. The disordered (Hxa−)2 dimer in two-fold orientation manifests its double-proton transfer process above Tc, whereas these protons are ordered in the ferroelectric phase. The H2-TPPZ2+ dication acts as a proton sponge by forming two intramolecular N···H+···N hydrogen bridges between the pyridyl units with a very short N···N distance. The dication in the paraelectric state adopts a nonpolar geometry due to the delocalization of the protons over two sites in the respective N···H+···N bonds. Below Tc, only one of the two protons gets localized, and the resultant acentric H2-TPPZ2+ ion generates the dipole moment responsible for the ferroelectricity.