Optical absorption and electron paramagnetic resonance (EPR) have been applied in parallel to investigate ${\mathrm{LiNbO}}_3$:Cr[0.1%] and ${\mathrm{LiNbO}}_3$:Cr[0.1%], Mg[X%] crystals (X=2, 4, 5, 5.5, and 6). From the data obtained, it has been concluded that ${\mathrm{Cr}}^{3+}$ ions occupy both ${\mathrm{Li}}^{+}$ and ${\mathrm{Nb}}^{5+}$ sites. In Mg undoped crystals the optical absorption and EPR spectra are attributed to ${\mathrm{Cr}}^{3+}$ ions in ${\mathrm{Li}}^{+}$ site. In crystals with Mg doping levels greater than 4.6 mol % both ${\mathrm{Li}}^{+}$ and ${\mathrm{Nb}}^{5+}$ sites are occupied, their relative concentration being governed by the ${\mathrm{Mg}}^{2+}$ 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 ${\mathrm{Cr}}^{3+}$ ions in ${\mathrm{Nb}}^{5+}$ sites is discussed considering the different models related to the nonstoichiometry of ${\mathrm{LiNbO}}_3$. © 1996 The American Physical Society.

• Received 9 February 1996

DOI:https://doi.org/10.1103/PhysRevB.54.13042