Identification of a Pb-related Ti3+ center in flux-grown KTiOPO4

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) have been used to identify a new Ti3+ center in KTiOPO4 crystals c ontaining lead impurities. Many of the K+ vacancies in this set of KTP crys tals are compensated nonlocally by Pb2+ ions substituting for K+ ions. Duri ng exposure to ionizing radiation (either 60 kV x rays or 355 nm photons fr om a tripled Nd:YAG laser), "free" electrons are trapped an Ti4+ ions near isolated Pb2+ ions, thus forming the perturbed Ti3+ ions observed with EPR and ENDOR. Four distinct Pb-related Ti3+ centers are formed by a 77 K irrad iation, but only one remains after a 5 min anneal at 180 K. This latter def ect, labeled the [Ti3+-Pb2+](A) center, is thermally unstable above 250 K. Angular dependence data were used to determine the g matrix, one Pb-207 hyp erfine matrix, and two P-31 hyperfine matrices for the [Ti3+-Pb2+](A) cente r. More generally, we note that oxygen-vacancy-associated Ti3+ centers coul d not be formed in these Pb-containing KTP crystals. Replacing oxygen-vacan cy-associated Ti3+ centers with the less stable [Ti3+-Pb2+] centers is expe cted to increase the gray-track resistance of KTP crystals used to generate the second harmonic of high power, near-infrared lasers. (C) 2000 American Institute of Physics. [S0021-8979(00)04722-8].