Luminescence of LiNbO3 : MgO,Cr crystals under high pressure

The results of high-pressure studies of LiNbO3:Cr(0.2%), Mg crystals doped with two concentrations of magnesium (2% and 5.5%) are reported. The results reveal information about the electronic structure of different Cr3+ centers in lithium niobate crystals. There are three major Cr3+ centers (denoted by alpha, beta, and gamma) in the crystal with 2% of magnesium. These centers correspond to Cr3+ ions in Li+ sites with different crystal field. At ambient pressure the ct center experiences strong crystal field and the beta and gamma centers are the intermediate crystal-field centers. The energy differences between the T-4(2) and E-2 levels are positive and negative for the beta and the gamma 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 delta) correponds to Cr3+ ions located in Nb5+ sites. The R lines of the Cr3+ centers exhibit very large redshift with pressure of about 3 cm(-1)/kbar. [S0163-1829(99)12331-2].