Structure and energetics of a ferroelectric organic crystal of phenazine and chloranilic acid

TL;DR

This study uses first-principles calculations with vdW-DF2 functional to analyze the structure, energetics, and ferroelectric properties of a phenazine-chloranilic acid crystal, revealing proton transfer as key to ferroelectricity.

Contribution

It provides detailed computational insights into the ferroelectric mechanism of an organic crystal, highlighting the role of proton transfer and improved treatment of van der Waals interactions.

Findings

Proton transfer from chloranilic acid to phenazine is energetically favorable.

Energy barrier of 8 meV exists between paraelectric and ferroelectric states.

No unstable phonon mode associated with proton transfer was found.

Abstract

We report first-principles calculations for a ferroelectric organic crystal of phenazine and chloranilic acid molecules. Weak intermolecular interactions are properly treated by using a second version of van der Waals density functional known as vdW-DF2 [K. Lee et al., Phys. Rev. B 82, 081101 (2010)]. Lattice constants, total energies, spontaneous electric polarizations, phonon modes and frequencies, and the energy barrier of proton transfer are calculated and compared with PBE and experiments whenever possible. We show that the donation of one proton from a chloranilic acid molecule to a neighboring phenazine molecule is energetically favorable. This proton transfer is the key structural change that breaks the centrosymmetry and leads to the ferroelectric structure. However, there is no unstable phonon associated with the proton transfer, and an energy barrier of 8 meV is found between the paraelectric and ferroelectric states.