Francisite: a model antiferroelectric?

No elementary antiferroelectric material (as defined by Charless Kittel in the 1950s) has been found experimentally to date, as far as we know. In this work, our colleagues measure an antiferroelectric mode softening in the Francisite Cu₃Bi(SeO₃)₂O₂Cl. This cell-doubling transition is measured with inelastic x-ray scattering and Raman spectroscopy finding the possibly first ever archetypal antiferroelectric with displacive character. We provide first-principles calculations that support the existence of a leading antipolar instability in the high-symmetry structure (with a related polar instability very close in energy).