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  • Title: Proton dynamics and room-temperature ferroelectricity in anilate salts with a proton sponge.
    Author: Horiuchi S, Kumai R, Tokunaga Y, Tokura Y.
    Journal: J Am Chem Soc; 2008 Oct 08; 130(40):13382-91. PubMed ID: 18781747.
    Abstract:
    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 T(c) = 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-TPPZ(2+) 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 T(c), whereas these protons are ordered in the ferroelectric phase. The H2-TPPZ(2+) 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 T(c), only one of the two protons gets localized, and the resultant acentric H2-TPPZ(2+) ion generates the dipole moment responsible for the ferroelectricity.
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