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.