Substitution of Er, In, and Hf in LiNbO3 : Evidence for multiple defect distributions about dopant sites

Abstract

Many dopants are incorporated into ${\mathrm{LiNbO}}_{3}$ (LNO) to modify the physical properties, particularly many optical properties. To understand these changes, the substitution site for the dopant must be known. Here we provide a detailed EXAFS study of the local environment about the metal (M) ion dopants indium, erbium, and hafnium in LNO, using crystals grown in both stoichiometric and congruent forms. In primarily substitutes at the Li site in congruent material; Er also substitutes mostly on the Li site with a small fraction roughly 10% on the Nb site---the substitution sites for Er do not appear to change much between stoichiometric and congruent LNO. For Hf, approximately half the dopant substitutes on Li and half on Nb. In all cases, some charge compensating defects are required (self-compensating schemes are not consistent with the data), and can occupy several nearby sites in the crystal leading to many slightly different distributions, some of which may be metastable. For each dopant, changes in the the M-O shell approximately follow the ionic radius of the dopant ion; in addition, the Nb neighbors about M on a Li site are shifted away from the dopant, with large displacements for some dopants such as Er. Comparisons with other experiments and existing calculations are discussed.