Electron paramagnetic resonance and electron–nuclear double-resonance study of Ti³⁺ centres in KTiOPO₄

Electron paramagnetic resonance and electron–nuclear double resonance have been used to characterize four Ti³⁺ centres in undoped crystals of potassium titanyl phosphate (KTiOPO₄ or KTP). These 3d¹ defects (S = 1/2) are produced by ionizing radiation (either 60 kV x-rays or 355 nm photons from a tripled Nd:YAG laser), and form when the regular Ti⁴⁺ ions in the crystal trap an electron. Two of these trapped-electron centres are only observed in hydrothermally grown KTP and the other two are dominant in flux-grown KTP. Both of the Ti³⁺ centres in hydrothermally grown crystals have a neighbouring proton (i.e. an OH⁻ molecule). In the flux-grown crystals, one of the Ti³⁺ centres is adjacent to an oxygen vacancy and the other centre is tentatively attributed to a self-trapped electron (i.e. a Ti³⁺ centre with no stabilizing entity nearby). The g matrix and phosphorus hyperfine matrices are determined for all four Ti³⁺ centres, and the proton hyperfine matrix is determined for the two centres associated with OH⁻ ions. These Ti³⁺ centres contribute to the formation of the grey tracks often observed in KTP crystals used to generate the second harmonic of high-power, near-infrared lasers.