Electron paramagnetic resonance study of an Al-associated oxygen hole center in kTiOPO₄:Al crystals

Electron paramagnetic resonance (EPR) has been used to study an oxygen hole centre (O^-) produced in Al-doped KTiOPO₄ crystals γ-irradiated at 300 K. The EPR spectrum exhibited by this point defect can be observed at temperatures between 300 and 100 K. A six-line hyperfine splitting which is ascribed to one ²⁷Al nucleus and a doublet splitting which is ascribed to one ³¹P nucleus have been observed at 150 K. At 150 K, the principal g values are 2.0027, 2.0157 and 2.0464, the principal values of the ²⁷Al hyperfine splitting are 6.3, 7.4 and 7.5 G and the principal values of the ³¹P hyperfine splitting are 0.6, 2.5 and 2.6 G. The hole is trapped at one of the titanyl oxygens and locally associated with an Al³⁺ impurity located at or near a Ti⁴⁺ site. The Al impurity stabilizes the oxygen hole center, which would not be stable at room temperature in undoped crystals.