Defects in inorganic photorefractive materials and their investigations

Defects in inorganic photorefractive materials and their investigations 25
Analogous MCD-ODMR studies have been performed with KTaO 3 :Ni [142].
Three low symmetry defects in various charge states were found.
6 Defects in the sillenites Bi 12 MO 20
(BMO, M = Si, Ge, Ti)
In their review article, Arizmendi et al. [144] drew attention to the lack of
knowledge concerning the nature and role of defects in these materials. The
situation has been clarified to a large extent, new spectroscopic evidence being
the main object of this Section.
6.1 Intrinsic defects in undoped crystals
Fig. 11. Upper part (arbitrary
ordinate scale):
Bleached state absorption
of BGO at 80 K (dashed line)
and additional absorption
(10.7 cm −1 at maximum)
induced by an illumination
with blue light (solid grey
line). Lower parts: CD and
MCD spectra (1.4 K) in the
bleached state (dotted lines)
andinthecoloredstate(solid
lines).
The most precise information has been obtained by combined optical
absorption, MCD and ODMR studies, performed with thermally bleached
(e.g., 1/2 hour at 500 ◦ C) as well as optically colored crystals in Paris
[145, 146, 147, 148, 149, 150] and in Osnabrück [149, 151]. Several important
conclusions are summarized in Ref. [152] and in a forthcoming article
[153]. As illustrated in the central part of Fig. 11 for BGO as an example,
an undoped and thermally bleached BMO sample does not exhibit any MCD
signal in the near-IR to UV spectral range. This means that the ’shoulder’ observed
near 3 eV in the corresponding absorption spectrum (top of Fig. 11) is
necessarily correlated with a diamagnetic defect. The ’shoulder’ has obviously
a counterpart in the natural circular dichroism spectrum (CD, ’0’ in Fig. 11)
with a minimum at the same energy. Illuminating the bleached BMO crystal
with light energies lying within the range of this CD band, causes a slight decrease
of its amplitude. Simultaneously intense additional absorption (bands