The results of high-pressure studies of ${\mathrm{LiNbO}}_3:\mathrm{C}\mathrm{r}(0.2\%)\mathrm{},$ Mg crystals doped with two concentrations of magnesium (2% and 5.5%) are reported. The results reveal information about the electronic structure of different ${\mathrm{Cr}}^{3+}$ centers in lithium niobate crystals. There are three major ${\mathrm{Cr}}^{3+}$ centers (denoted by α, β, and γ) in the crystal with 2% of magnesium. These centers correspond to ${\mathrm{Cr}}^{3+}$ ions in ${\mathrm{Li}}^{+}$ sites with different crystal field. At ambient pressure the α center experiences strong crystal field and the β and γ centers are the intermediate crystal-field centers. The energy differences between the ${}^4{T}_2$ and ${}^{2}E$ levels are positive and negative for the β and the γ centers, respectively. Additional broadband luminescence observed in the sample with 5.5% of magnesium even at pressure of almost 100 kbar testifies that another very weak crystal-field center exists in this crystal. This center (denoted by δ) correponds to ${\mathrm{Cr}}^{3+}$ ions located in ${\mathrm{Nb}}^{5+}$ sites. The R lines of the ${\mathrm{Cr}}^{3+}$ centers exhibit very large redshift with pressure of about 3 ${\mathrm{cm}}^{\mathrm{-}1}$/kbar.

• Received 8 December 1998

DOI:https://doi.org/10.1103/PhysRevB.60.7707