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

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 387 nm and a photoluminescence excitation (PLE) band peaking near 273 nm 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 630 nm. 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.