Copper-doped lithium triborate (LiB3O5) crystals: A photoluminescence, thermoluminescence, and electron paramagnetic resonance study

Abstract

When doped with copper ions, lithium borate materials are candidates for use in radiation dosimeters. Copper-doped lithium tetraborate (Li2B4O7) crystals have been widely studied, but little is known thus far about copper ions in lithium triborate (LiB3O5) crystals. In the present investigation, Cu+ ions (3d10) were diffused into an undoped LiB3O5 crystal at high temperature. These ions occupy both Li+ and interstitial positions in the crystal. A photoluminescence (PL) band peaking near 387nm and a photoluminescence excitation (PLE) band peaking near 273nm verify that a portion of these Cu+ ions are located at regular Li+ sites. After an irradiation at room temperature with x rays, electron paramagnetic resonance (EPR) spectra show that Cu+ ions at Li+ sites have trapped a hole and converted to Cu2+ ions (3d9) while Cu+ ions at interstitial sites have trapped an electron and converted to Cu0 atoms (3d104s1). Two distinct Cu2+ trapped-hole spectra are formed by the x rays: one due to isolated Cu2+ ions with no nearby defects and the other due to perturbed Cu2+ ions. When the x-ray-irradiated crystal is heated above room temperature, a thermoluminescence (TL) peak appears at 120°C with a maximum in the emitted light near 630nm. EPR shows that this TL peak occurs when trapped electrons are thermally released from interstitial Cu0 atoms. Thermal quenching above room temperature prevents the electron-hole recombination at Cu2+ ions from contributing to the TL emission.