Electron paramagnetic resonance of copper centres in potassium tantalate single crystals

Electron paramagnetic resonance (EPR) of exchange-coupled pairs of Cu2+ ions was observed in KTaO3:Cu, K1-xLixTaO3:Cu and K1-xLixTa1-yNbyO3:Cu single crystals. The spectra are typical of centres with S = 1 and tetragonal symmetry with the pair axis parallel to the [100], [010] or [001] directions. The angular and temperature dependences of the signals have been studied. The experimental results are explained in terms of a pair centre model, which consists of a chain of two equivalent copper ions and three oxygen vacancies. The two copper ions occupy neighbouring tantalum sites and. one of the oxygen vacancies lies between them, the chain running in the (100) direction. Ferromagnetic exchange coupling takes place between the copper ions, as confirmed by the thermal behaviour of the corresponding EPR spectra.