The locations of Cr3+ centers were investigated with the help of electron nuclear double resonance methods. A detailed study of the spectra was carried out for a large set of crystals having various Cr concentrations and [Li]/[Nb] ratios. For the main axial Cr13+ center, a full investigation of the electron nuclear double resonance angular dependences was performed; yielding the parameters of hyperfine and quadrupole interactions. It was found that Cr3+ substituted for Li+. However, the Cr ion was shifted by 0.02nm from the normal Li site. An analysis of the quadrupole splitting of 53Cr showed that the parameter of the axial crystal field was negative (-0.387/cm at 4.2K). The determined parameters of the hyperfine interactions were several times larger than the calculated classical dipole-dipole interactions. The data permitted the reconstruction of the shape and width of the electron paramagnetic resonance line in perfect crystals. The difference between the calculated and observed electron paramagnetic resonance linewidth could be used to estimate non-stoichiometric crystal imperfection. Rather high values of the isotropic hyperfine interactions indicated the transfer of electron density to neighboring nuclei. An analysis of the spectra of satellite centers, Cr23+-Cr03+, showed that Cr3+ also substituted for Li+ in these centers. There were strong distortions of the electron density distributions, caused by the presence of an intrinsic defect in the surroundings of the Cr. A model in which vNb was a charge compensator of a CrLi center explained many of the details of the measured spectra.

ENDOR Study of Cr3+ Centers Substituting for Lithium in Lithium Niobate. G.Malovichko, V.Grachev, A.Hofstaetter, E.Kokanyan, F.Scharmann, O.Schirmer: Physical Review B, 2002, 65[22], 224116 (11pp)