Spectroscopy of near-stoichiometric LiNbO3:MgO,Cr crystals under high pressure

The results of high-pressure spectroscopic studies of near-stoichiometric LiNbO3:Cr(0.1%) and LiNbO3:MgO,Cr(0.1%) crystals are reported. Much lower inhomogeneous broadening of electronic transitions in near-stoichiometric crystals than in congruent ones allows us to obtain better resolution of the spectroscopic measurements. The results of measurements show that Cr3+ ions occupy mostly the same centers in near-stoichiometric crystals and in the congruent ones. In near-stoichiometric crystals doped only with chromium, me Cr3+ ions substitute the Li+ sites in the LiNbO3 host forming one major center (denoted as center gamma) and two lower concentration centers, denoted by alpha and beta. In near-stoichiometric crystals doped additionally with a proper amount of MgO, the additional low-strength crystal-field center delta dominates the spectrum. This center corresponds to the Cr3+ ions in Nb5+ sites. The hydrostatic pressure above 75 kbar transforms this center into the high-strength crystal-field center. Due to better resolution of measurements, it was possible to determine the splitting of the E-2 level for the delta center, which is equal to about 17 cm(-1). Estimated values of the energy difference between the position of the T-4(2) and the E-2 levels for centers beta, gamma, and delta in these near-stoichiometric LiNbO3 crystals are equal to -20, -242, and more than -1000 cm(-1), respectively.