ON THE LATTICE SITE OF TRIVALENT DOPANTS AND THE STRUCTURE OF MG2+-OH-(-)M3+ DEFECTS IN LINBO3-MG CRYSTALS

The lattice sites of a number of trivalent co-dopants (In, Nd, Gd, Er, Tm, and Lu) have been determined in LiNbO3 crystals doped also with Mg, using the Rutherford back-scattering and proton-induced x-ray emission channelling techniques. In, Er and Lu have been found mainly on Li sites, but a small fraction of In and Lu may substitute for Nb ions. Gd and Tm ions also replace mainly Li, but some of these ions seem to be present in other positions, possibly related to some kinds of aggregate, e.g. self-compensating pairs. A fraction of Nd ions probably occupies interstitial sites in structurally empty oxygen octahedra, while other Nd ions may substitute for host cations. In addition to the already known effect of Cr, Fe and Nd ions, In, Lu and also Sc give rise to an OH- absorption band between 3499 and 3523 cm-1, which is attributed to Mg2+(Li site)-OH-(O site)-M3+(Nb site) complex defects. No such band is observed for Y, Gd, Fr and Tm co-doped crystals.