OPTICAL-ABSORPTION STUDIES OF TETRAGONAL AND CUBIC THIN-FILM YTTRIA-STABILIZED ZIRCONIA

Optical absorption studies of oxidized and H-2-reduced tetragonal and cubic zirconia films (75 less-than-or-equal-to d less-than-or-equal-to 2100 nm), YSZ single crystals and TZP ceramics (50 less-than-or-equal-to d less-than-or-equal-to 200-mu-m) have been performed in order (i) to provide reliable band gap values of these commonly used fast oxygen ion conductors and (ii) to investigate the type and extent of structural disorder introduced by the aliovalent dopant Y2O3 and the electronic charge carriers. At r.t. the optical band gaps of TZP (3 mol% Y2O3) and YSZ (8 mol% Y2O3) differ only slightly: 5.7 eV +/- 0.1 eV and 5.8 eV +/- 0.05 eV, respectively. Their temperature coefficients are almost-equal-to -4 x 10(-4) eV/K. As indicated by the pronounced Urbach-type band edge absorption, fairly wide band tail states exist in both materials as a consequence of structural disorder. The continuous change of the Urbach slope with the yttria content or oxygen-vacancy-defect concentration, respectively, demonstrates that these sub-gap states mainly arise from oxygen vacancy defects, V(o). Upon reduction with H2, localized electron states are formed at the expense of these tail states.