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

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 F₀ complex consisting of an 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 F₀^* 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 F₀^* state, at which point the corresponding F centers will decay by losing an electron to one of the F₀^* 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.