MG-DOPING THRESHOLD EFFECT AND H-CONTAINING DEFECTS IN LINBO3

The OH infrared absorption spectra of heavily Mg-doped LiNbO3 codoped with a trivalent or a divalent cation impurity have been investigated, and some correlated with ESR measurements. We examine how the thresho ld effect in Mg doping determines the incorporation sites of the codop ant and attempt to derive a general relationship between impurity latt ice sites and the OH spectrum. A main absorption band that peaked at 3 536 cm-1 appeared in all the samples and is attributed to OH stretchin g vibration in the Mg(Li)-OH-Mg(Nb) complex. Those codoped with M3+ (M = Fe, Cr, Nd) exhibited additional OH bands within the 3500-3525 cm-1 range, the actual peak position depending on the charge misfit and th e ionic radius of the impurity cation. They are attributed to MgLi-OH- MeNb3+ complexes. We did not find any additional band for M3+ - Er+, H o3+, Tm3+. A simple scheme is proposed that depicts whether such a new feature should appear, depending on the ionic radius and the electron egativity of the trivalent ion. We observed no change in either the in frared or the EsR Spectrum in the case of a Tr2+ codopant (Tr = Co, Ni or Cu), implying that its lattice site is unaffected by Mg doping.