Soft optical phonon in ferroelastic BiVO₄

We report the observation of an underdamped q∼O soft optical phonon in the Raman spectra of the paraelastic and ferroelastic phases of BiVO4. This mode has the same symmetry as the ferroelastic strain. The temperature dependence of the soft optical phonon energy indicates that the ferroelastic transition is continuous and that the order parameter has a Landau-type behavior over a wide temperature range. © 1977.