On the room temperature ferroelectricity of hydrogen-bonded charge transfer crystals

We present a theoretical investigation of the anomalous ferroelectricity of mixed-stack charge transfer molecular crystals, based on the Peierls-Hubbard model, and first principles calculations for its parameterization. This approach is first validated by reproducing the temperature-induced transition and the electronic polarization of TTF-CA, and then applied to a novel series of hydrogen-bonded crystals, for which room temperature ferroelectricity has recently been claimed. Our analysis shows that the hydrogen-bonded systems present a very low degree of charge transfer and hence support a very small polarization. A critical re-examination of experimental data supports our findings, shedding doubts on the ferroelectricity of these systems. More generally, our modelling allows the rationalization of general features of the ferroelectric transition in charge transfer crystals, and suggests design principles for materials optimization.