ENDOR study ofCr3+centers substituting for lithium in lithium niobate

Abstract

The surroundings of several ${\mathrm{Cr}}^{3+}$ centers in lithium niobate crystals were investigated with the help of electron nuclear double resonance (ENDOR). In order to find optimal conditions for the ENDOR observation a detailed study of these spectra was carried out for a large set of crystals with different chromium concentrations and [Li]/[Nb] ratios. For the main axial ${\mathrm{Cr}}_{1}^{3+}$ center a full investigation of the ENDOR angular dependencies was performed and the parameters of hyperfine and quadrupole interactions were determined. It is found that ${\mathrm{Cr}}^{3+}$ substitutes for ${\mathrm{Li}}^{+},$ however, the chromium ion is shifted by 0.2 \AA{} from the regular Li site. An analysis of quadrupole splitting of ${}^{53}\mathrm{Cr}$ shows that the parameter of the axial crystal field is negative: ${b}_{2}^{0}=\ensuremath{-}0.387{\mathrm{cm}}^{\ensuremath{-}1}$ at 4.2 K. The determined parameters of the hyperfine interactions are several times larger than calculated classical dipole-dipole interactions. The obtained data allowed us to reconstruct the shape and width of the electron paramagnetic resonance (EPR) line in the perfect crystals. The difference between the calculated and observed EPR linewidth can therefore be used for the estimation of the nonstoichiometric crystal imperfection. Rather high values of isotropic (contact) hyperfine interactions demonstrate a transfer of electron density to neighboring nuclei. An analysis of the ENDOR spectra of satellite centers ${\mathrm{Cr}}_{2}^{3+}\ensuremath{-}{\mathrm{Cr}}_{9}^{3+}$ has shown that ${\mathrm{Cr}}^{3+}$ substitutes for ${\mathrm{Li}}^{+}$ in these centers also, however, there are strong distortions of electron density distributions caused by the presence of an intrinsic defect in the chromium surroundings. A model with ${v}_{\mathrm{Nb}}$ as a charge compensator of ${\mathrm{Cr}}_{\mathrm{Li}}$ center explains most of details of both EPR and ENDOR spectra in a natural way.