| Abstract
| - Centrosymmetric pyrazinium NH+···N bonded complexes allow their dielectric properties to beanalyzed separately from the bulk ferroelectric polarization observed in the noncentrosymmetric DABCOmonosalts. The method of dielectric permittivity measurements has been employed for monitoringpolarization fluctuations generated by proton transfers in the NH+···N bonded linear polycations. Therevealed dielectric response of [C4H5N2]+BF4- and [C4H5N2]+ClO4- cannot be reconciled with thecentrosymmetric symmetry of their structures, but suggests formation of polar defects or nanoregions. Uniquetransformations of the pyrazine [C4H5N2]+BF4- complex between linear NH+···N hydrogen-bondedpolycationic aggregates antiparallel in phase γ, perpendicular chains in phase β, and with NH+···N bondsbroken in phase α have been observed. The [C4H5N2]+ClO4- and [C4H5N2]+BF4- complexes as grown at290 K are isostructural in orthorhombic phase γ, space group Pbcm, with linear polycationic chains arrangedantiparallel. On heating, the tetrafluoroborate and perchlorate salts each undergoes two first-order phasetransitions at similar temperatures about 340−360 K, while on cooling only one phase transition has beenobserved. An extremely unique sequence of two phase transitions subsequently lowering the symmetry ofthe [C4H5N2]+BF4- crystal when temperature is increased has been evidenced: the orthorhombic phase γheated above 343 K transforms into the monoclinic C2/c-symmetric phase β, in which the NH+···N bondedlinear chains assume perpendicular arrangement; and at about 353−357 K the anions and cations adopta typical ionic-crystal packing without homonuclear NH+···N hydrogen bonds in a still lower-symmetrymonoclinic P21/n-symmetric structure. The exceptional perpendicular arrangement of the linear NH+···Nbonded chains in phase β constitutes a unique system where polarization of small regions can assumevarious orientations within a plane, depending on the H+ sites. No phase transitions or anomalous dielectricresponse were observed in the NH+···O bonded [C4H5N2]+NO3- complex. The unprecedented structure−property relations of the pyrazinium complexes fully confirm the role of the NH+···N bond transformationsfor the dielectric response of analogous DABCO ferroelectrics.
|
