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

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 (Formula presented) niobium vacancy (Formula presented) and oxygen interstitial ions (Formula presented) belong to (Formula presented) or (Formula presented) point group symmetries; this is also valid for the complexes with niobium substitution and the (Formula presented) antisite as the compensator (Formula presented) Clusters of an oxygen vacancy and an impurity on a niobium site ((Formula presented) configurations) have (Formula presented) symmetry only, but never (Formula presented) 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 (Formula presented) 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 — (Formula presented) and niobium vacancy (Formula presented) in the first or further neighboring shells. Two (Formula presented)’s compensate five (Formula presented) 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. © 1999 The American Physical Society.