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Optical properties of LiNbO3 : Cr crystals co-doped with Germanium oxide

Camarillo, E. and Murrieta, H. and Hernandez, J.M. and Zoilo, R. and Flores, M.C. and Han, T.P.J. and Jaque, F. (2008) Optical properties of LiNbO3 : Cr crystals co-doped with Germanium oxide. Journal of Luminescence, 128 (5-6). pp. 747-750. ISSN 0022-2313

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Lithium niobate (LiNbO3) crystals doped with chromium ions show a clear green colouring reflecting the absorption profile of the dominating [Cr](Li) defect centres. A significant change in its colouration takes place when it is co-doped with other valency impurities such as Mg2+, Sc3+ and W6+, above a certain threshold concentration. This concentration singularity has been attributed to the formation of [Cr](Nb) centres coexisting with the [Cr](Li) centres. In this work, we extended the investigation on the effect of co-dopant ions in Cr:LiNbO3 to tetravalent cation such as GeO2. A singularity in the relative intensity of the (4)A(2)-> T-4(1) and (4)A(2)-> T-4(2) absorption band was observed for a concentration of similar to 1.5 mol%, compared with 4.5 mol% for Mg2+. The photoluminescence emission spectra also reveal a new emission band, at a lower energy than the [Cr](Li) centre, corresponding to this threshold concentration. A charge compensation model is proposed to explain the role of cation impurities and results are compared with those of other valence impurities.