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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34 B. Briat et al.<br />
7.6 Bismuth tellurite Bi 2 TeO 5 (CaF 2 -based lattice)<br />
At high write beam <strong>in</strong>tensities, this relatively new material is announced to<br />
be competitive for data storage with the known best <strong>materials</strong> (LiNbO 3 :Fe or<br />
BaTiO 3 ), with the advantage of the self-fix<strong>in</strong>g of volume holograms [203, 204,<br />
205].<br />
Bi 2 TeO 5 crystallizes <strong>in</strong> an orthorhombic structure with a large number<br />
(∼ 17%) of structural oxygen vacancies to compensate for the negative charge<br />
of the Bi 3+ ions with respect to Te 4+ . It is an <strong>in</strong>terest<strong>in</strong>g host for dopants<br />
s<strong>in</strong>ce it has three different cation sites with different co-ord<strong>in</strong>ation spheres,<br />
Bi(1) with 8, Bi(2,3) with 7, <strong>and</strong> Te with 5 oxygens; a good <strong>in</strong>corporation was<br />
observed for Cr, V, Mo, substitut<strong>in</strong>g for Te, <strong>and</strong> rare earth ions substitut<strong>in</strong>g<br />
for Bi [206]. The exposure of Bi 2 TeO 5 :Cr crystals to white light <strong>in</strong>duces photochromism.<br />
The partial transformation of Cr 6+ <strong>in</strong>to Cr 5+ was <strong>in</strong>ferred from<br />
photoabsorption experiments [207] but no EPR result has yet been reported.<br />
8 Hydrogen<br />
Just as the defects treated before, hydrogen is a tool to tailor the <strong>photorefractive</strong><br />
properties of a material as well as to <strong>in</strong>vestigate its properties. This<br />
element is present <strong>in</strong> all compounds conta<strong>in</strong><strong>in</strong>g oxygen as a constituent, with<br />
concentrations rang<strong>in</strong>g from a few ppm up to several percent, after special<br />
treatments depend<strong>in</strong>g on the crystal. The presence of hydrogen is not always<br />
an advantage, for example <strong>in</strong> fibers. Therefore procedures have been <strong>in</strong>vented<br />
to reduce hydrogen, <strong>in</strong> particular for LiNbO 3 [208, 209]. Usually hydrogen is<br />
bonded to an oxygen ion <strong>and</strong> performs a vibration, referred to as OH-stretch<br />
mode, with an energy of about 3200-3700 cm −1 . Detailed spectroscopic data<br />
have been compiled <strong>and</strong> reported recently [210]. Besides basic studies of the<br />
spectroscopic properties of the OH-stretch mode, it served very often as a<br />
tool to <strong>in</strong>vestigate properties of the host. Because of its easy observation by<br />
absorption spectroscopy a qualitative <strong>and</strong> <strong>in</strong> favorable cases even a quantitative<br />
measure of the hydrogen content is possible. Aga<strong>in</strong> LiNbO 3 served as an<br />
example. It is found that the OH absorption is polarized perpendicularly to<br />
the c-axis of LN <strong>in</strong> accordance with the microscopic model of proton <strong>in</strong>corporation<br />
established by ENDOR, see Section 4. An absorption strength per ion<br />
of about 9 × 10 −18 cm was found (for details of the def<strong>in</strong>ition see [211, 212]).<br />
By monitor<strong>in</strong>g the change of the hydrogen isotope concentrations <strong>in</strong> thermal<br />
processes the diffusivity of these isotopes can be determ<strong>in</strong>ed. In the last two<br />
decades strong efforts have been devoted to study the diffusion of hydrogen<br />
isotopes <strong>in</strong> <strong>in</strong>sulat<strong>in</strong>g <strong>materials</strong>, especially <strong>in</strong> oxides, see [213] <strong>and</strong> references<br />
there<strong>in</strong>.<br />
Protons can be <strong>in</strong>volved <strong>in</strong> thermal fix<strong>in</strong>g of holograms (LN, KTN, BSO),<br />
however Buse et al. concluded from spatially resolved optical data especially<br />
for LiNbO 3 that <strong>their</strong> presence is not m<strong>and</strong>atory [214]. Strongly related to