Polar Instability in Ferroelastic Bismuth Vanadate BiVO₄

Bismuth vanadate BiVO₄ is known as a model ferroelastic crystal displaying a second-order transition where shear strain is the primary order parameter. More recently, it has attracted attention for its optical properties, but also for its potential antiferroelectric character, a property that might in fact be common to all proper or pseudo-proper ferroelastics. In this work, we report a combined theoretical and experimental study of the low frequency polar modes in BiVO₄. Infrared spectroscopy and inelastic neutron scattering are used to measure the polar mode at the Brillouin zone center and follow its dispersion as a function of temperature. We demonstrate the existence of a strong coupling between the polar mode and the primary order parameter that causes the crystal to display a dielectric anomaly typical of antiferroelectric systems. We also discuss how the polar instability is reflected from first-principles calculations, and how polar displacements of Bismuth produce unsual phonon-phonon coupling in the inelastic neutron spectra.