Electronic structure and electron energy-loss spectroscopy ofZrO2zirconia

Abstract

The atomic and electronic structures of zirconia are calculated within density functional theory, and their evolution is analyzed as the crystal-field symmetry changes from tetrahedral [cubic $(c\text{\ensuremath{-}}{\mathrm{ZrO}}_{2})$ and tetragonal $(t\text{\ensuremath{-}}{\mathrm{ZrO}}_{2})$ phases] to octahedral (hypothetical rutile ${\mathrm{ZrO}}_{2}$), to a mixing of these symmetries (monoclinic phase, $m\text{\ensuremath{-}}{\mathrm{ZrO}}_{2}$). We find that the theoretical bulk modulus in $c\text{\ensuremath{-}}{\mathrm{ZrO}}_{2}$ is 30% larger than the experimental value, showing that the introduction of yttria in zirconia has a significant effect. Electronic structure fingerprints which characterize each phase from their electronic spectra are identified. We have carried out electron energy-loss spectroscopy experiments at low momentum transfer and compared these results to the theoretical spectra calculated within the random phase approximation. We show a dependence of the valence and $4p$ (${N}_{2,3}$ edge) plasmons on the crystal structure, the dependence of the latter being brought into the spectra by local-field effects. Last, we attribute low energy excitations observed in EELS of $m\text{\ensuremath{-}}{\mathrm{ZrO}}_{2}$ to defect states $2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the top of the intrinsic valence band, and the EELS fundamental band gap value is reconciled with the 5.2 or $5.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ gaps determined by vacuum ultraviolet spectroscopy.