An Antipolar-driven Ferroelectric: Phase Transition of Cu-Cl Boracite

Materials belonging to the boracite family with the chemical formula M3B7O13X, where M is a metal and X a halide, are interesting for their functional properties, in particular ferroelectricity, pyroelectricity and multiferroicity. Cu-Cl boracite is particularly interesting due to its unconventional electrostatic properties. Indeed, charged domain walls form spontaneously on cooling through the phase transition. They exhibit a changed conductivity compared to the bulk [1]; even more intriguingly, they move anomalously under the application of an electric field in a way consistent with the existence of local negative capacitance [2].

Here, we will explore the phase transition of Cu-Cl properties that lead to the spontaneous existence of charged walls. We presente results of single crystal X-ray diffraction and dielectric permittivity in the light of group theory. We will particularly show that an antipolar phonon mode drives the improper ferroelastic and ferroelectric phase transition and detail the unique coupling between spontaneous strain and polarisation.
From this, we will develop an understanding of the functional response of Cu-Cl boracite to an electrical stimulus and in particular its dielectric and piezoelectric properties. This response will be put in perspective with the electrostatic energy increase associated with the observed negative capacitance.