  Bismuth vanadate (BiVO4) has recently been under focus for its potential use
in photocatalysis thanks to its well-suited absorption edge in the visible
light range. Here, we characterize the optical absorption of a BiVO4 single
crystal as a function of temperature and polarization direction by reflectance
and transmittance spectroscopy. The optical band gap is found to be very
sensitive to temperature, and to the monoclinic-to-tetragonal ferroelastic
transition at 523K. The anisotropy, as measured by the difference in absorption
edge for light polarized parallel and perpendicular to the principal axis, is
reduced from 0.2 eV in the high-temperature tetragonal phase to 0.1 eV at
ambient temperature. We show that this evolution is dominantly controlled by
the ferroelastic shear strain. These findings provide a route for further
optimization of bismuth-vanadate-based light absorbers in photocatalytic
devices.
