Structural Assignment of Polarization in Hydrogen-Bonded Supramolecular Ferroelectrics

A 1:1 adduct of phenazine (Phz) with chloranilic acid (H 2 ca) and bromanilic acid (H 2 ba), in which nonpolar neutral molecules are connected by the intermolecular O−H···N hydrogen bonds, shows ferroelectric transitions. Neutron diffraction analysis of these materials reveals that there are two inequivalent hydrogen-bonding sites below the ferroelectric transition temperature (253 K for Phz-H 2 ca and 168 K for Phz-H 2 ba). This asymmetry was also observed by the maximum entropy method (MEM) analysis of the synchrotron X-ray diffractions. The structure analysis results suggest that the origin of the polarization of the material is the symmetry breaking based on the two inequivalent hydrogen bonds.