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 complex consisting of an ion and an oxygen vacancy. We assume the ground state of this complex lies in the valence band, whereas its first excited state 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 state, at which point the corresponding F centers will decay by losing an electron to one of the 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.
- Received 21 June 1989
DOI:https://doi.org/10.1103/PhysRevB.40.8555
©1989 American Physical Society