Ferroelectricity inNH⋯NHydrogen Bonded Crystals

$\mathrm{NH}\ensuremath{\cdots}\mathrm{N}$ hydrogen bonded crystals of $[{\mathrm{C}}_{6}{\mathrm{H}}_{7}{\mathrm{N}}_{2}{]}^{+}\ifmmode \dot{}\else \.{}\fi{}{\mathrm{ClO}}_{4}^{\phantom{\rule{0ex}{0ex}}\ensuremath{-}}$ and $[{\mathrm{C}}_{6}{\mathrm{H}}_{7}{\mathrm{N}}_{2}{]}^{+}\ifmmode \dot{}\else \.{}\fi{}{\mathrm{BF}}_{4}^{\phantom{\rule{0ex}{0ex}}\ensuremath{-}}$ undergo ferroelectric phase transitions at 377.0(5) and 378.1(5) K, respectively. Owing to particularly simple structure, built of linear $\mathrm{NH}\ensuremath{\cdots}\mathrm{N}$ bonded aggregates, the spontaneous polarization of the crystals can be straightforwardly calculated from ionic displacements. The $\mathrm{NH}\ensuremath{\cdots}\mathrm{N}$ bonded structures have been compared with transformations of analogous bistable $\mathrm{OH}\ensuremath{\cdots}\mathrm{O}$ hydrogen bonds in ${\mathrm{H}}_{2}\mathrm{O}$ ices and ${\mathrm{KH}}_{2}{\mathrm{PO}}_{4}$-type ferroelectrics.