Quantized ferroelectricity in multivalent ion conductors with non-polar point groups.

Ferroelectricity with switchable polarizations is generally associated with small ion-displacements and occurs only in 10 specific polar point groups, which is not a necessary requirement for ion conduction where the ions can also be electrically displaced but by much longer distances. Herein, through first-principles calculations, we predict the formation of unconventional ferroelectricity based on previous experimental reports on topotactic reaction with an aliovalent cation between trigonal layers of ion conductors. In such systems, the multivalent cations are surrounded by vacant sites that can simultaneously migrate by a much larger distance compared with conventional displacive ferroelectricity, giving rise to a quantized change in polarization even if the crystal lattices do not belong to the 10 polar groups. The deviation from classical principles can be attributed to the long ion displacements in ferroelectric ion conductors during switching that can lead to the transformation between multiple equivalent symmetrical stable states, which cannot be realized by the relatively small ion displacements in current ferroelectrics. The evenly distributed vacant sites due to Coulomb repulsion do not break the insulativity of the systems, while their inhomogeneous distribution under an electric field or in ferroelectric domain walls will give rise to high electrical conductance, which may be utilized for constructing nanoscale artificial ionic synapses that enable neuromorphic computing.