High-pressure studies were made of samples which were doped with 0.2%Cr, and 2 or 5.5%Mg. The results revealed that there were 3 major Cr3+ centers (α, β, γ) in crystals which contained 2%Mg. These centers corresponded to Cr3+ ions on Li sites, with differing crystal fields. At ambient pressures, the α center experienced a strong crystal field. The β and γ centers were intermediate crystal-field centers. The energy differences between the 4T2 and 2E levels were positive and negative for the β and γ centers, respectively. An additional broad-band luminescence which was observed in samples with 5.5%Mg, even at pressures of almost 100kbar, indicated that another very weak crystal-field center existed in such crystals. This center (δ) corresponds to Cr3+ ions which were located on Nb5+ sites. The R-lines of the Cr3+ centers exhibited very large red-shifts, with pressure, of about 3/cm-kbar.

Luminescence of LiNbO3:MgO,Cr Crystals under High Pressure. A.Kamińska, J.E.Dmochowski, A.Suchocki, J.Garcia-Sole, F.Jaque, L.Arizmendi: Physical Review B, 1999, 60[11], 7707-10