Anisotropy of band-to band transitions and polarons in BiVO4 studied by resonance Raman spectroscopy

Bismuth vanadate (BVO) is a potential anti-ferroelectric crystal relevant for water splitting applications. The monoclinic crystal structure makes is highly anisotropic, and little is understood about its electronic band structure and role of defect states. We study these by analyzing the intensity of Raman modes as a function of excitation energy, which increases when excitation energy approaches a real transition level. We find two types of resonances, the states near at the band gap and well below the band gap. The band gap states occur near 2.4 eV and show an anisotropic behavior between light polarized a and c crystal axes, with the energy difference of 50 meV. We reported similar anisotropy using the optical transmission data. The second type of resonances occur at ~0.3eV below the band gap. We attribute this peak to the polarons, defect states stabilized by phonon polarization and find the coupling between the defect and the phonons to be higher for the low energy phonons compared to the high energy ones. The polaritonic states show orientation anisotropy, where higher resonant Raman intensity occurs for light polarized along certain crystal directions, indicating the orientation of dipole moment of the defect state. Our study shines light on the electronic properties of oxides and the role of phonons in defect states stabilization.