Observation of paramagnetic point defects in BBO (β-BaB2O4) crystals

Abstract The electron paramagnetic resonance (EPR) technique has been used to investigate point defects in single crystals of BBO (β-BaB 2 O 4 ). This borate compound is a widely used nonlinear optical material, especially in the ultraviolet region, and point defects are expected to play a role in its device performance. There were no observable EPR spectra in our as-grown BBO crystals. However, an irradiation at room temperature with X-rays produced two stable EPR signals. One of these spectra, containing ten hyperfine lines when the magnetic field is parallel to the crystal's c axis, is assigned to a hole trapped on an oxygen ion adjacent to a barium vacancy. The ten lines are due to interactions with the two neighboring 11 B nuclei. The other radiation-induced spectrum consists of a single line with an effective g value of 2.017 when the magnetic field is along the c axis. We tentatively assign this spectrum to Ni + ions substituting for Ba 2+ ions. In separate experiments, it was found that heating an as-grown BBO crystal to 760 °C for several hours produced an EPR spectrum due to Cu 2+ ions. Subsequent exposure at room temperature to X-rays reduces the intensity of the Cu 2+ spectrum.