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 containing lead impurities. Many of the K⁺ vacancies in this set of KTP crystals are compensated nonlocally by Pb²⁺ ions substituting for K⁺ ions. During exposure to ionizing radiation (either 60 kV x rays or 355 nm photons from a tripled Nd:YAG laser), "free" electrons are trapped on 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 irradiation, but only one remains after a 5 min anneal at 180 K. This latter defect, labeled the [Ti³⁺-Pb²⁺]_A center, is thermally unstable above 250 K. Angular dependence data were used to determine the g matrix, one ²⁰⁷Pb hyperfine matrix, and two ³¹P hyperfine matrices for the [Ti³⁺-Pb²⁺]_A center. More generally, we note that oxygen-vacancy-associated Ti³⁺ centers could not be formed in these Pb-containing KTP crystals. Replacing oxygen-vacancy-associated Ti³⁺ centers with the less stable [Ti³⁺-Pb²⁺] centers is expected to increase the gray-track resistance of KTP crystals used to generate the second harmonic of high power, near-infrared lasers.