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 (H(2)xa). Dielectric measurements revealed phase transitions at T-c = 334 and 172 K for bromanilate (Hba(-)) and chloranilate (Hca(-)) salts, respectively. The room-temperature ferroelectricity of the (H-2-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 center dot center dot center dot H+center dot center dot center dot N bond of the H-2-TPPZ(2+) dication and in an O-H center dot center dot center dot O- hydrogen-bonded cyclic dimer of the ortho-quinoid Hxa(-) anions. The disordered (Hxa-)2 dinner in two-fold orientation manifests its double-proton transfer process above T., whereas these protons are ordered in the ferroelectric phase. The H-2-TPPZ(2') dication acts as a proton sponge by forming two intramolecular N center dot center dot center dot H+ center dot center dot center dot N hydrogen bridges between the pyridyl units with a very short N center dot center dot center dot 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 center dot center dot center dot H+ center dot center dot center dot N bonds. Below T-c, only one of the two protons gets localized, and the resultant acentric H-2-TPPZ(2+) ion generates the dipole moment responsible for the ferroelectricity.