Ferroelectric order in highly disordered molecular-ionic crystals

The ordering of the anionic sublattice at the coexisting high orientational disorder of the cationic sublattice produces spontaneous polarization in imidazolium tetrafluoroborate $[{\mathrm{C}}_{3}{\mathrm{N}}_{2}{\mathrm{H}}_{5}{]}^{+}[{\mathrm{BF}}_{4}{]}^{\ensuremath{-}}.$ This ferroelectric crystal exhibits the Curie point at 297.0 K and undergoes two other polymorphic phase transitions at 213.6 and 187.8 K. In spite of its pentagonal ring the dynamically disordered polar cation is localized at a special position on the threefold axis of the highly symmetrical rhombohedral unit cell. Thus it does not contribute immediately to the spontaneous polarization which originates predominantly from the behavior of the distorted tetrahedral fluoroborate anion.