Electron paramagnetic resonance study of electron and hole traps in β-BaB2O4 crystals

Electron paramagnetic resonance (EPR) has been used to investigate point defects in single crystals of β-BaB2O4 (commonly referred to as BBO). An irradiation with x rays at 77 K produces two dominant EPR spectra, one electronlike and the other holelike. The trapped-electron center exhibits a 16-line hyperfine pattern from two boron nuclei. With the magnetic field parallel to the c axis, its g value is 1.9993 and the B11 hyperfine splittings are 49 and 42 G. We suggest that this defect is an oxygen vacancy with a trapped electron nearly equally shared by the two neighboring boron ions. In contrast, the trapped-hole center exhibits a four-line hyperfine pattern due to one boron, and is assigned to a hole localized on a nonbridging oxygen ion with no other defects nearby (i.e., a self-trapped hole). With the magnetic field along the c axis, this center has a g value of 2.0113 and a B11 hyperfine splitting of 14.1 G. Warming the crystal to temperatures between 80 and 90 K destroys the initial four-line hole spectrum and introduces other hole centers (most likely perturbed by nearby Na+ ions substituting for Ba2+ ions or by barium vacancies). These electron and hole traps are expected to play a role in the response of BBO crystals to high-power pulsed ultraviolet lasers.