Ferroelectricity inNH⋯NHydrogen Bonded Crystals

Abstract

$\mathrm{NH}\cdots \mathrm{N}$ hydrogen bonded crystals of $[{\mathrm{C}}_6{\mathrm{H}}_7{\mathrm{N}}_2{]}^{+}̇{\mathrm{ClO}}_4^{-}$ and $[{\mathrm{C}}_6{\mathrm{H}}_7{\mathrm{N}}_2{]}^{+}̇{\mathrm{BF}}_4^{-}$ undergo ferroelectric phase transitions at 377.0(5) and 378.1(5) K, respectively. Owing to particularly simple structure, built of linear $\mathrm{NH}\cdots \mathrm{N}$ bonded aggregates, the spontaneous polarization of the crystals can be straightforwardly calculated from ionic displacements. The $\mathrm{NH}\cdots \mathrm{N}$ bonded structures have been compared with transformations of analogous bistable $\mathrm{OH}\cdots \mathrm{O}$ hydrogen bonds in ${\mathrm{H}}_2\mathrm{O}$ ices and ${\mathrm{KH}}_2{\mathrm{PO}}_4$-type ferroelectrics.