Spectroscopic investigation of the electronic structure of yttria-stabilized zirconia

The electronic structure and optical properties of yttria-stabilized zirconia are investigated as a function of the yttria content using multiple experimental and theoretical methods, including electron energy-loss spectroscopy, Kramers-Kronig analysis to obtain the optical parameters, photoelectron spectroscopy, and density functional theory. It is shown that many properties, including the band gaps, the crystal field splitting, the so-called defect gap between acceptor $({\mathrm{Y}}_{\mathrm{Zr}}^{{}^{\ensuremath{'}}})$ and donor $({\mathrm{V}}_{\mathrm{O}}^{\ifmmode\bullet\else\textbullet\fi{}\ifmmode\bullet\else\textbullet\fi{}})$ states, as well as the index of refraction in the visible range exhibit the same ``zig-zag-like'' trend as the unit cell height does, showing the influence of an increased yttria content as well as of the tetragonal--cubic phase transition between $8\phantom{\rule{4pt}{0ex}}\mathrm{mol}%$ and $20\phantom{\rule{4pt}{0ex}}\mathrm{mol}%\phantom{\rule{0.16em}{0ex}}{\mathrm{Y}}_{2}{\mathrm{O}}_{3}$. Also, with \ifmmode \check{C}\else \v{C}\fi{}erenkov spectroscopy (CS), a new technique is presented, providing information complementary to electron energy-loss spectroscopy. In CS, the \ifmmode \check{C}\else \v{C}\fi{}erenkov radiation emitted inside the TEM is used to measure the onset of optical absorption. The apparent absorption edges in the \ifmmode \check{C}\else \v{C}\fi{}erenkov spectra correspond to the energetic difference between the disorder states close to the valence band and the oxygen-vacancy-related electronic states within the band gap. Theoretical computations corroborate this assignment: they find both, the acceptor states and the donor states, at the expected energies in the band structures for diverse yttria concentrations. In the end, a schematic electronic structure diagram of the area around the band gap is constructed, including the chemical potential of the electrons obtained from photoelectron spectroscopy. The latter reveal that tetragonal YSZ corresponds to a $p$-type semiconductor, whereas the cubic samples exhibit $n$-type semiconductor properties.