Defects in inorganic photorefractive materials and their investigations

24 B. Briat et al.
photorefractive properties of KNbO 3 [140]. Besides isolated Fe 3+
Nb
and Fe3+
Nb
-V O several low symmetry associations of Fe 3+
Nb
with unknown partners are
reported. As grown, oxidized and reduced samples were investigated. Lightinduced
changes of EPR and optical absorption were observed in some cases,
but only in one case with the same defect. Definite relations between Fe-defects
and the photorefractive behavior of the crystals could not be established. Nor
was it possible to decide experimentally whether Fe 2+ in KNbO 3 absorbs at
2.55 eV [140]. With EPR/optical absorption it was established that the optical
absorption of Fe 2+ has its peak at 2.1 eV in BT [115], at 1.9 eV in BCT [141]
and at 1.5 eV in KTaO 3 [142] (Fig. 10).
1400
1000
wavelength (nm)
700 500
400
cubic Fe 3+
tagged MCD /E(arb.units)
3+
Fe - VO
rhombic Fe 3+
3+
Fe - OI
4+
Fe - OI
cubic Fe 2+
2+
Fe - VO
1 1.5 2 2.5 3 3.5
photon energy E (eV)
Fig. 10. MCD spectra of various
Fe-containing defects in KaTO 3:Fe
[143] assigned to the indicated
species by ODMR.
In view of the obstacles to obtain detailed EPR information about defects
in KNbO 3 , it was decided to investigate the similar KTaO 3 :Fe as a model
system. Such crystals are cubic at all temperatures and thus facilitate the
investigation by ODMR-MCD. At least eleven different Fe containing defects,
most of them low symmetry associations, were found [143]. Fig. 10 shows
a selection among them, characterized by their MCD spectra. Usually such
bands are overlapping in the same crystal; with the ODMR technique it is
possible to select those bands originating from a definite defect. Since the
frequency ranges of the MCD structures are identical to those of the related
absorption bands, this plot shows at what energies to expect absorption bands
of the given defect configurations and charge states, also in the similar KNbO 3 .