MgO codoping-induced change in the site distribution of Cr3+ ions in LiNbO3

Optical absorption and electron paramagnetic resonance (EPR) have been applied in parallel to investigate LiNbO3:Cr[0.1%] and LiNbO3:Cr[0.1%], Mg[X%] crystals (X = 2, 4, 5, 5.5, and 6). From the data obtained, it has been concluded that Cr3+ ions occupy both Li+ and Nb5+ sites. In Mg undoped crystals the optical absorption and EPR spectra are attributed to Cr3+ ions in Li+ site. In crystals with Mg doping levels greater than 4.6 mol % both Li+ and Nb5+ sites are occupied, their relative concentration being governed by the Mg2+ content. The double occupancy affects the overall absorption spectrum, giving rise to reproducible color changes in the crystal. Finally, the threshold found for the occupation of Cr3+ ions in Nb5+ sites is discussed considering the different models related to the nonstoichiometry of LiNbO3.