Local environment of Mn2+ and Cr3+ centres in LiNbO3: Discussion of recent ENDOR results

Recent electron nuclear double resonance (ENDOR) experiments on Mn 2+ in LiNbO 3 allowed to identify the on-axis Nb neighbours together with several Li shells with one nearest neighbour Limissing, but failed to resolve the nearest off-axis Nb shell. This failure was attributed to local disorder. We discuss here that the nearby Li vacancy may be filled by a Nb which has relaxed from its normal lattice site, while Mn 2+ remains on a Li site. In the case of Cr 3+ in heavily Mg-codoped LiNbO 3 substitution was shown to be on the Nb site. This assignement and its dependence on Mg is confirmed by a comparison of the 53 Cr quadrupole and ESR data in LiNbO 3 and Al 2 O 3