Anomalous Protonic-Glass Evolution from Ordered Phase in NH···N Hydrogen-Bonded DabcoHBF4 Ferroelectric

Abstract

Dielectric properties, spontaneous polarization, and phase transitions of the NH+...N bonded ferroelectric dabcoHBF4 (i.e., 1,4-diazabicyclo[2.2.2]octane tetrafluoroborate, [C6H13N2]+ BF4-) have been related to one-dimensional arrangement of the cations and to their conformational properties. The onset of conformational transformation lowering the symmetry of the cations, rearrangement of the anions, and proton disordering in NH+...N hydrogen bonds, linking the cations into linear chains, lead to a ferroelectric-ferroelectric phase transition at T23 = 153 K. A weak coupling between the protonic and anionic sites in dabcoHBF4 results in the formation of distinct phase-diagram regions: the high-temperature paraelectric phase with disordered protons, the intermediate ferroelectric phase with the protons ordered, and the low-temperature ferroelectric phase where the protons become disordered again. The lowest temperature phase remains ferroelectric owing to the ionic displacements, while the protons assume the glass state. In this phase the H+ transfers involve local formation of neutral, monocationic, and dicationic species. Such an anomalous formation of protonic glass state from the ordered phase depends on the subtle structural features pertaining to the proton transfers in bistable hydrogen bonds. In paraelectric phase I, between the mp and T12 = 374 K, the anions are orientationally disordered, the protons are disordered in the hydrogen bonds and the cations rotate about the [z] direction; in ferroelectric phase II below T12, the protons and cations order, the dabco cations assume a planar conformation of ethylene bridges, and the anions exhibit a residual temperature-dependent gradual ordering (two 80:20 occupied sites of the anion are still observed at 332 K); and in ferroelectric phase III below T23, the cations assume left- and right-twisted propeller conformations and the anions are ordered but the protons become disordered in the hydrogen bonds.