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

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 d i- or trivalent impurities, if they substitute For a lithium ion or are inc orporated in a structural vacancy, whereas the antisite defect, niobium on a lithium site, can serve as charge compensation only for impurity ions rep lacing niobium. From a comparison with experimental data ii is concluded th at intrinsic defects in the first to ninth shells around the impurity are r esponsible for clearly distinguishable satellite EPR lines (local charge co mpensation). Small perturbations of the axial crystal field, caused by dist ant defects, are revealed only in the observed line asymmetries and broaden ing.