Electron and hole traps in Ag-doped lithium tetraborate (Li2B4O7) crystals

Abstract

Electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and thermoluminescence (TL) are used to characterize the primary electron and hole trapping centers in a lithium tetraborate (Li2B4O7) crystal doped with Ag. Three defects, two holelike and one electronlike, are observed after exposure at room temperature to 60 kV x-rays. The as-grown crystal contains both interstitial Ag+ ions and Ag+ ions substituting for Li+ ions. During the irradiation, substitutional Ag+ ions (4d10) trap holes and two distinct Ag2+ centers (4d9) are formed. These Ag2+ EPR spectra consist of doublets (i.e., the individual 107Ag and 109Ag hyperfine lines are not resolved). One of these hole centers is an isolated unperturbed Ag2+ ion and the other is a Ag2+ ion with a nearby perturbing defect. EPR and ENDOR angular-dependence data provide the g matrix and the 107Ag and 109Ag hyperfine matrices for the more intense isolated hole center. In contrast, the electronlike EPR spectrum produced during the irradiation exhibits large nearly isotropic hyperfine interactions with 107Ag and 109Ag nuclei and a neighboring I = 3/2 nucleus (either 7Li or 11B). This spectrum is assigned to a trapped electron shared between an interstitial Ag ion and the substitutional I = 3/2 ion. Upon warming, the radiation-induced trapped electrons and holes seen with EPR recombine between 100 and 200 °C, in agreement with a single strong TL peak observed near 160 °C.