Ferroelectricity Driven by Orbital Resonance of Protons in CH$_3$NH$_3$Cl and CH$_3$NH$_3$Br

TL;DR

This study reveals that ferroelectricity in methylammonium halides arises from proton orbital resonance and magnetic interactions, offering a new pathway to discover organic ferroelectrics.

Contribution

It demonstrates that proton orbital magnetic moments induce ferroelectricity in methylammonium halides through orbital resonance and lattice distortion.

Findings

Ferroelectric phases identified via pyroelectric and dielectric measurements.

Magnetic susceptibility shows discontinuities at Curie temperatures.

Structural instabilities are linked to magnetic origin.

Abstract

The $\beta$ and $\gamma$ phases of methylammonium chloride CH$_3$NH$_3$Cl and methylammonium bromide CH$_3$NH$_3$Br are identified to be ferroelectric $via$ pyroelectric current and dielectric constant measurements. The magnetic susceptibility also exhibits pronounced discontinuities at the Curie temperatures. We attribute the origin of spontaneous polarization to the emergence of two groups of proton orbital magnetic moments from the uncorrelated motion of the CH$_3$ and NH$_3$ groups in the $\beta$ and $\gamma$ phases. The two inequivalent frameworks of intermolecular orbital resonances interact with each other to distort the lattice in a non-centrosymmetric fashion. Our findings indicate that the structural instabilities in molecular frameworks are magnetic in origin as well as provide a new pathway toward uncovering new organic ferroelectrics.