Defects in inorganic photorefractive materials and their investigations
Defects in inorganic photorefractive materials and their investigations
Defects in inorganic photorefractive materials and their investigations
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<strong>Defects</strong> <strong>in</strong> <strong>in</strong>organic <strong>photorefractive</strong> <strong>materials</strong> <strong>and</strong> <strong>their</strong> <strong>in</strong>vestigations 11<br />
Fig. 5. The various optical absorption states of reduced LN. Absorption b<strong>and</strong> correlated<br />
with a: Nb 4+<br />
Nb<br />
free polarons [29]. b: polarons localized as Nb4+<br />
Li<br />
c: bipolarons<br />
bound as Nb 4+<br />
Li<br />
-Nb 4+<br />
Nb<br />
. B<strong>and</strong> c has been observed with a congruent crystal after<br />
reduction for 1 h at 900 ◦ C <strong>in</strong> the dark; b<strong>and</strong> b after illum<strong>in</strong>at<strong>in</strong>g this crystal with<br />
the unfiltered light of a xenon arc at 80 K. B<strong>and</strong> a results from reduc<strong>in</strong>g a congruent<br />
crystal, doped with 6% Mg, for 9 h at 500 ◦ C.<br />
an <strong>in</strong>tervalence transition from Nb 4+<br />
Li<br />
to Nb 5+ , i.e. from a localized level to<br />
Nb<br />
Fig. 6. Schematic models of the three Nb 4+ conta<strong>in</strong><strong>in</strong>g defects <strong>in</strong> LN. a: The groundstate<br />
orbital of Nb 4+<br />
Li<br />
. b: Groundstate orbital of Nb4+<br />
Nb<br />
. c: Two electrons with antiparallel<br />
sp<strong>in</strong>s trapped at a preformed Nb Li -Nb Nb pair. By relax<strong>in</strong>g towards each<br />
other, <strong>in</strong>dicated by the double arrows, the covalent bond between both orbitals is<br />
strengthened. In this way the electronic energy, driv<strong>in</strong>g the bipolaron formation (see<br />
Sec. 2), is lowered.