Resonant Raman scattering and other new coupling phenomena in ferroelastic BiVO4

With the introduction of ferroic oxides into the research field of photovoltaic and photocatalytic applications, an increasing interest in understanding the coupling mechanisms of light with the electronic bands in this class of materials has been developed over the last years. In many of these examples, reliable band structure calculations are not yet available mostly because of the complexity in the crystal structure which demands for many experimental evidences to verify even basic characteristics of the structure. Also classical experimental techniques based on the optical absorption seem to reach their limits here resulting often in controversial publications on the nature of the band gap.
The goal of this work is to exploit resonant Raman scattering to reveal information on the electronic band structure and thereby create a better understanding of the light-matter coupling mechanisms in ferroic oxides. As a model material, we choose to work on ferroelastic bismuth vanadate, which is known for its second-order phase transition from the high-symmetry tetragonal to the low-symmetry monoclinic phase.
On the way to understand resonant Raman scattering in bismuth vanadate, i.e. the coupling of lattice vibrations and electronic transitions, we extensively studied the phonons and optical properties and discovered new coupling phenomena of various kinds. We found a phonon-phonon coupling in the lattice vibrations of the vanadium oxygen tetrahedron that are strongly temperature and polarization dependent. The coupling strength is of opposite sign depending on the polarization conditions and diminishes at the structural phase transition at which one of the phonon modes changes its symmetry.
Additionally, the transmission data evidence the coupling between the spontaneous shear strain and the electronic structure that primarily defines the strong temperature dependence of the optical absorption in monoclinic bismuth vanadate.
Further, we report on resonant Raman scattering effects that allow us to study the coupling between phonons and electrons. We succeeded to measure multiple resonant Raman bands corresponding to not only the band gap at 2.4 eV but also to a polaronic defect level at 2.0 eV. The coupling strength to the electronic states in the conduction band and in the defect level varies between the Raman modes corresponding to different lattice vibrations. By varying the light polarization in the experiment, we could access another band-to-band transition that lies 120 meV above the band gap energy. This energy difference matches the optical anisotropy quantified by transmission measurements.
With this work we demonstrate that resonant Raman spectroscopy is a very powerful tool to study not only structural but also electronic properties in ferroic oxides. It has the potential to go deeper in the analysis on the electronic band structure than classical techniques such as UV/Vis spectroscopy or ellipsometry.