Alignment of axial iron complexes in KTaO3, induced by anisotropic
photo ionization with polarized light
Photo recharging was found of tetragonal Fe3+K
- Oi centers in incipient ferroelectric
KTaO3 crystals that clearly reveals itself in reduced samples
where these centers are absent in thermal equilibrium and appear only after
irradiation in the visible due to photo recharging from another charge
state. Low energy threshold for the creation of Fe3+K
- Oi centers was determined at 2.05eV. It
was shown that the concentrations of photo produced centers with different
orientations depend on the polarization of incident light, thus resulting
in an alignment of the centers in the EPR-detectable charge state. The
direction of the alignment changes in the course of irradiation for the
case of reduced sample. As a result of the alignment, macroscopic symmetry
of the system “host lattice + Fe centers” lowers from cubic
to tetragonal.
A simple model allowed to explain all the experimental findings. The
model is based on anisotropic photo charge transfer between FeK
- Oi centers and both host electron bands
but does not involve real reorientations of the centers. Solutions to the
system of kinetic equations which was written on the base of the model
allowed for a quantitative description of spectral, polarization and kinetic
dependencies of concentrations of Fe3+K
- Oi centers with different orientations
in reduced and oxidized samples, including the change of the sign of the
alignment in reduced samples.
The mechanism of the alignment of anisotropic defect center resulting
from anisotropic photo recharging of the centers seems to be rather general.
It involves no microscopic mechanisms specific for the studied system but
only photoinduced charge exchange between defect centers and host lattice.
This inevitably happens when incident photon energy exceeds a certain threshold.
Generally speaking, for anisotropic defect centers the corresponding cross-sections
are always anisotropic. However, the magnitude of the effect depends on
many microscopic parameters.
We gratefully acknowledge the supports of Russian Foundation for Basic
Research (no.96-02-17972) and of DFG-RFBR (no. 436 RUS 113/341/1).
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