A systematic analysis and classification of clusters which consisted of impurity (extrinsic) and intrinsic defects in crystal, was performed in order to understand the main features of the observed electron paramagnetic resonance spectra. It was shown that possible configurations for Li substitution with charge compensators such as a Li vacancy (XLi-YLi), Nb vacancy (XLi-YNb) or O interstitial ion (XLi-Yi) belonged to the C3 or C1 point-group symmetries. This was also true of complexes with Nb substitution and for the NbLi antisite as the compensator (XNb-YLi). Clusters involving an O vacancy and an impurity on a Nb (XNb-YO configuration) had C1 symmetry only, and never C3 symmetry. A detailed study of electron paramagnetic resonance spectra for a wide set of crystals with various Cr concentrations and [Li]/[Nb] ratios was carried out. As well as the main axial Cr3+ center, 8 satellite Cr centers were experimentally resolved and the parameters of their spin Hamiltonians were determined by fitting the angular dependences of electron paramagnetic resonance lines. It was found that, in stoichiometric material, less Cr was incorporated into the crystal and that the satellite centers disappeared. A correlation between electron paramagnetic resonance, optical absorption and luminescence spectra was observed and analyzed. The existence of the family of Cr centers was explained in terms of a common hypothesis concerning charge compensation by intrinsic defects. Using a minimal model, which was sufficient to explain all of the experimental data, it was assumed that the satellite centers included 2 defects, CrLi and the Nb vacancy, VNb, in the first or distant neighboring shells. Two VNb defects compensated five CrLi defects. In conventional congruent crystals, the relative concentrations of additional satellite centers would be comparable to the concentration of the main center. It was therefore concluded here that both kinds of center were equally responsible for many of the properties of this niobate.

Axial and Low-Symmetry Centers of Trivalent Impurities in Lithium Niobate: Chromium in Congruent and Stoichiometric Crystals. G.Malovichko, V.Grachev, E.Kokanyan, O.Schirmer: Physical Review B, 1999, 59[14], 9113-25