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

An EPR study has revealed light-induced recharging and optical alignment of the Fe _Ta ⁴⁺ -V _O 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 Fe _K ³⁺ -O_i center. These two centers were established to undergo mutual charge transfer, in which the electron released in the photoionization of the Fe _K ²⁺ -O_i center is trapped by the Fe _Ta ⁵⁺ -V _O center. Irradiation by light with a photon energy below 2.05 eV, which is the ionization threshold of Fe _K ²⁺ -O_i, 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 Fe _Ta ⁴⁺ -V _O and Fe _K ³⁺ -O_i 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 Fe_Ta-V _O and Fe_K-O_i complexes, gives rise, nevertheless, to the alignment of the centers along (or at right angles to) the light polarization vector.