Unusual Ferroelectricity of Trans-Unitcell Ion-Displacement and Multiferroic Soliton in Sodium and Potassium Hydroxides

We show the first-principles evidence of a hitherto unreported type of ferroelectricity with ultra-long ion-displacement in sodium and potassium hydroxides. Even a small amount of proton vacancies can completely change the mode of proton-transfer from intra-unitcell to trans-unitcell, giving rise to multiferroic soliton with mobile magnetism and a tremendous polarization that can be orders of magnitude higher compared with most perovskite ferroelectrics. Their vertical polarizations of thin-film are robust against depolarizing field, rendering various designs of 2D ferroelectric field-transistors with non-destructive readout and ultra-high on/off ratio via sensing the switchable metallic/insulating state.