• Katrusiak, Andrzej
• Szafrański, Marek

Abstract

<p>Centrosymmetric pyrazinium NH⁺-⋯N bonded complexes allow their dielectric properties to be analyzed separately from the bulk ferroelectric polarization observed in the noncentrosymmetric DABCO monosalts. The method of dielectric permittivity measurements has been employed for monitoring polarization fluctuations generated by proton transfers in the NH⁺⋯N bonded linear polycations. The revealed dielectric response of [C₄H₅N₂]⁺BF ^4- and [C₄H₅N²] ⁺ClO₄^- cannot be reconciled with the centrosymmetric symmetry of their structures, but suggests formation of polar defects or nanoregions. Unique transformations of the pyrazine [C ₄H₅N₂]¹BF₄^- complex between linear NH⁺⋯N hydrogen-bonded polycationic aggregates antiparallel in phase γ, perpendicular chains in phase β, and with NH⁺⋯N bonds broken in phase α have been observed. The [C₄H₅N₂]⁺ClO ₄^- and [C₄H₅N₂] ⁺BF₄^- complexes as grown at 290 K are isostructural in orthorhombic phase γ, space group Pbcm, with linear polycationic chains arranged antiparallel. On heating, the tetrafluoroborate and perchlorate salts each undergoes two first-order phase transitions at similar temperatures about 340-360 K, while on cooling only one phase transition has been observed. An extremely unique sequence of two phase transitions subsequently lowering the symmetry of the [C₄H₅N ₂]⁺BF₄^- crystal when temperature is increased has been evidenced: the orthorhombic phase γ heated above 343 K transforms into the monoclinic C2/c-symmetric phase β, in which the NH ⁺⋯N bonded linear chains assume perpendicular arrangement; and at about 353-357 K the anions and cations adopt a typical ionic-crystal packing without homonuclear NH⁺⋯N hydrogen bonds in a still lower-symmetry monoclinic P2₁/n-symmetric structure. The exceptional perpendicular arrangement of the linear NH⁺⋯N bonded chains in phase β constitutes a unique system where polarization of small regions can assume various orientations within a plane, depending on the H⁺ sites. No phase transitions or anomalous dielectric response were observed in the NH⁺⋯O bonded [C₄H₅N ₂]⁺NO₃^- complex. The unprecedented structure-property relations of the pyrazinium complexes fully confirm the role of the NH⁺⋯N bond transformations for the dielectric response of analogous DABCO ferroelectrics. © 2006 American Chemical Society.</p>

Topics

• impedance spectroscopy
• phase
• Hydrogen
• phase transition
• defect
• space group