Proton-displacive ferroelectricity in neutral cocrystals of anilic acids with phenazine

Abstract

Hydrogen bonding in solids has provided diverse attractive issues. Prof. C. N. R. Rao and his collaborators have made representative achievements on experimental and theoretical grounds as well as on the resultant crystal engineering and functionalities. Many of the key issues clarified in those extensive studies are valid in our investigations on ferroelectricity of hydrogen-bonded supramolecules, linear chain cocrystals of phenazine (Phz) with anilic acids (H₂xa). Ferroelectrics have a spontaneous polarization that is switchable by an applied electric field, and have long constituted numerous technological applications such as capacitors, sensors, and information storage devices. In this paper, we overview the dielectric, ferroelectric, structural, and optical properties of such a family of hydrogen-bonded cocrystals as complemented with some latest advances to disclose the key experimental findings. Excellent ferroelectric properties are highlighted by the large response under small electric field in spite of the use of originally neutral and nonpolar constituents. The ferroelectric phase transition asymmetrically elongates an O–H bond with the strengthened O–H⋯N intermolecular bond. This mechanism is distinct from the site-to-site collective transfer of protons observed in conventional ferroelectrics like KH₂PO₄ (KDP). Rather, it represents quite an exotic proton displacement, being an incipient phenomenon of the transition from the neutral to the monovalent ionic form. Furthermore, the lower temperature regions find successive phase transitions to an additional ferroelectric phase having a doubled or threefold supercell. Periodic proton ordering is proposed as a possible picture of this ground state.