Color-center-induced band-gap shift in yttria-stabilized zirconia

Abstract

An increase of the room-temperature band gap from 4.23 to 4.96 eV is observed in crystals of the superionic material yttria-stabilized cubic zirconia (YSZ) when the crystals are reduced either electrolytically or in a hydrogen atmosphere. The original absorption edge of 4.23 eV in unreduced YSZ can be accounted for by the excitation of an $F_A$ complex consisting of an ${\mathrm{Y}}^{3+}$ ion and an $F^{+}$ oxygen vacancy. We assume the ground state of this complex lies in the valence band, whereas its first excited state $F_A^{\mathrm{*}}$ formed by adding an additional electron lies in the gap 0.73 eV below the conduction band; the observed absorption is then due to optical excitation of this state from the valence band. Reduction of YSZ leads to the formation of doubly occupied oxygen vacancies, i.e., F centers, giving rise to a band of states in the gap. Arguments are put forth to show that as the F-center concentration increases, the mean energy of this band is raised by F-F interactions or by changes in the lattice relaxation; eventually, part of the band will lie above the $F_A^{\mathrm{*}}$ state, at which point the corresponding F centers will decay by losing an electron to one of the $F_A^{\mathrm{*}}$ states. This results in a shift of the optical absorption edge to the true band-gap energy, i.e., 4.96 eV, which is a true band-to-band transition.