Anisotropic photoionization as a mechanism of axial iron center alignment in KTaO3

Abstract

An EPR study has revealed light-induced recharging and optical alignment of the FeTa4+-VO tetragonal complexes in KTaO3. The data on the optical creation and destruction of this center by light of different polarizations and wavelengths are discussed together with similar results obtained for the FeK3+-Oi center. These two centers were established to undergo mutual charge transfer, in which the electron released in the photoionization of the FeK2+-Oi center is trapped by the FeTa5+-VO center. Irradiation by light with a photon energy below 2.05 eV, which is the ionization threshold of FeK2+-Oi, reverses this process. In both cases, the absorption cross section depends on the orientation of the center axis relative to the light polarization vector. As a result, the FeTa4+-VO and FeK3+-Oi tetragonal centers in KTaO3 acted upon by polarized light undergo orientation-sensitive light-induced recharging and the defects with the given charge state are no longer characterized by an equally probable distribution of the orientations of their axes over the three 〈100〉 directions. This mechanism, which does not involve real reorientations of the FeTa-VO and FeK-Oi complexes, gives rise, nevertheless, to the alignment of the centers along (or at right angles to) the light polarization vector.