TI3- A PARAMAGNETIC JAHN-TELLER CENTER IN VACUUM-REDUCED LINBO3-MG-TISINGLE-CRYSTALS( ON NB SITE )

An axial EPR signal observed in vacuum annealed LiNbO3 single crystals doped with 8 mol % Mg and 0.05 mol% Ti has been attributed to Ti3+ on Nb site. The g tensor components (g(parallel to) = 1.760+/-0.005 and g(perpendicular to) = 1.786+/-0.005 for T = 5 K and g(parallel to) = g (perpendicular to) = 1.893+/-0.005 for T = 74 K), resulting in outstan dingly low values of the orbital reduction factor (from k = 0.16 to 0. 13), can be explained by a dynamic pseudo-Jahn-Teller effect. The vibr onic coupling of the center is much stronger than that reported for ot her nd(1) centers with similar trigonally distorted octahedral coordin ation in LiNbO3 like the Ti3+ on Li site and trapped Nb4+ polarons on both cation sites. Stronger coupling for ions on or near the Nb site i s understood as a result of smaller trigonal splitting of the ground s tate due to the more central position of the Nb site in the oxygen oct ahedron. Electron transfer from the observed Ti3+ center to lattice ni obiums, resulting in Nb4+ trapped polarons, has been stimulated by ill umination in the near UV region. The average energy difference of the involved Ti4+/3+ and Nb5+/4+ donor levels has been estimated to be 70/-23 meV depending also on the distance of compensating Mg2+ ions on n earby Li sites.