Axial and low-symmetry centers of trivalent impurities in lithium niobate: Chromium in congruent and stoichiometric crystals

Abstract

A systematic analysis and classification of clusters, consisting of impurity (extrinsic) and intrinsic defects in lithium niobate crystals, were made in order to understand the main features of the observed EPR spectra. It is shown that possible configurations for lithium substitution with charge compensators such as a lithium vacancy ${(X}_{\mathrm{Li}}{\ensuremath{-}Y}_{\mathrm{Li}}),$ niobium vacancy ${(X}_{\mathrm{Li}}{\ensuremath{-}Y}_{\mathrm{Nb}}),$ and oxygen interstitial ions ${(X}_{\mathrm{Li}}{\ensuremath{-}Y}_{i})$ belong to ${C}_{3}$ or ${C}_{1}$ point group symmetries; this is also valid for the complexes with niobium substitution and the ${\mathrm{Nb}}_{\mathrm{Li}}$ antisite as the compensator ${(X}_{\mathrm{Nb}}{\ensuremath{-}Y}_{\mathrm{Li}}).$ Clusters of an oxygen vacancy and an impurity on a niobium site (${X}_{\mathrm{Nb}}{\ensuremath{-}Y}_{\mathrm{O}}$ configurations) have ${C}_{1}$ symmetry only, but never ${C}_{3}$ symmetry. A detailed study of EPR spectra for a wide set of crystals with different chromium concentrations and [Li]/[Nb] ratios was carried out. Besides the main axial ${\mathrm{Cr}}^{3+}$ center, eight satellite chromium centers were experimentally resolved and parameters of their spin Hamiltonians were determined by fitting angular dependences of EPR lines. It was found that in stoichiometric material less chromium is incorporated into the crystal and that the satellite centers disappeared. A correlation of EPR, optical absorption, and luminescence spectra was observed and analyzed. The existence of the family of chromium centers was explained on the basis of one common hypothesis about charge compensation by intrinsic defects. In a minimal model, sufficient to explain all experimental data, it is assumed that the satellite centers include two defects --- ${\mathrm{Cr}}_{\mathrm{Li}}$ and niobium vacancy ${v}_{\mathrm{Nb}}$ in the first or further neighboring shells. Two ${v}_{\mathrm{Nb}}$'s compensate five ${\mathrm{Cr}}_{\mathrm{Li}}.$ Since in conventional congruent crystals the relative concentration of additional satellite centers is comparable with the concentration of the main center, the conclusion was made that both kinds of centers are equally responsible for many of the lithium niobate properties.