Models of axial and low-symmetry impurity centers in lithium niobate crystals derived from EPR and endor data

Abstract

Possible configurations of defect complexes in conventional lithium niobate crystals, consisting of an impurity associated with intrinsic defects, are analyzed. Lithium or niobium vacancies can compensate a charge excess of di- or trivalent impurities, if they substitute for a lithium ion or are incorporated in a structural vacancy, whereas the antisite defect, niobium on a lithium site, can serve as charge compensation only for impurity ions replacing niobium. From a comparison with experimental data it is concluded that intrinsic defects in the first to ninth shells around the impurity are responsible for clearly distinguishable satellite EPR lines (local charge compensation). Small perturbations of the axial crystal field, caused by distant defects, are revealed only in the observed line asymmetries and broadening.