Electron paramagnetic resonance study of Ag0 atoms and Ag2+ ions in β-BaB2O4 nonlinear optical crystals

Silver ions have been diffused into single crystals of β-BaB2O4 (commonly referred to as BBO). The crystals were wrapped in silver foil and then held in air at 760°C for periods of time ranging from 1 to 7 h. This treatment resulted in Ag+ ions occupying both Ba2+ sites and interstitial sites within the crystal. After the diffusion, the crystal was irradiated at room temperature with 60 kV x-rays. Three distinct paramagnetic defects, one holelike and two electronlike, were produced by the x-rays and observed with electron paramagnetic resonance (EPR) at temperatures between 15 and 80 K. The Ag+ ions substituting for Ba2+ ions trapped radiation-induced holes and became paramagnetic Ag2+ ions. At the same time, interstitial Ag+ ions trapped electrons and became paramagnetic Ag0 atoms. There are two distinct versions of these latter centres. The EPR spectra of the Ag0 atoms show resolved hyperfine from the 107,109Ag isotopes, as well as from a neighbouring nucleus (possibly a Na+ impurity substituting for a Ba2+ ion). All the radiation-induced EPR signals were stable at room temperature for hours.