The effect of the valence state upon the lattice location of impurities in single crystals was investigated by using hexavalent dopants (W6+, U6+) which were introduced by doping the congruent melt. The distribution coefficients were found, using Rutherford back-scattering spectrometry, to be equal to 0.2 for W and to 0.07 for U. Channelling studies showed that the hexavalent impurities were located on the Nb site in both cases. The preferential occupation of Nb sites suggested that the formation of Li vacancies was favoured. This was concluded to constitute strong evidence for the Li-vacancy defect model of congruent LiNbO3.

Incorporation of Hexavalent Impurities into LiNbO3. A.Kling, J.G.Margues, M.F.Da Silva, M.D.Serrano, E.Dieguez, J.A.Sanz-Garcia, J.Garcia-Sole, F.Agullo-Lopez, J.C.Soares: Radiation Effects and Defects in Solids, 1999, 150[1-4], 641-5