Photoluminescence from Ag²⁺ ions in lithium tetraborate (Li₂B₄O₇) crystals

Two broad photoluminescence (PL) bands peaking near 502 and 725 nm are observed at room temperature in Ag-doped Li₂B₄O₇ crystals pre-irradiated at room temperature with X-rays. Their respective excitation bands peak near 297 and 325 nm. The requirement that the crystal be pre-irradiated establishes that these emissions are not related to Ag⁺ ions. Electron paramagnetic resonance (EPR) helps assign models to the PL bands. EPR spectra show that two substitutional Ag²⁺ trapped-hole centers and one interstitial Ag⁰ trapped-electron center are produced at room temperature by the X-rays. A majority of the Ag²⁺ ions have no neighboring defects (and are referred to as Center A) while the remaining Ag²⁺ ions have a nearby defect (and are referred to as Center B). The PL bands are assigned to these two Ag²⁺ ions (the 502 nm emission to Center A and the 725 nm emission to Center B). A charge-transfer mechanism is responsible for the observed luminescence, i.e., an electron from a neighboring oxygen ion, upon excitation, moves to the Ag²⁺ ion and produces a Ag⁺ ion with an adjacent O⁻ ion. This excited-state complex quickly decays radiatively and the original Ag²⁺ and O²⁻ ions are restored. Optical absorption related to Ag²⁺ and Ag⁰ defects are observed in the ultraviolet and visible after the Ag-doped crystals are X-ray irradiated at room temperature. - Highlights: • Photoluminescence bands peaking near 502 and 725 nm are observed in Ag-doped Li₂B₄O₇. • A pre-irradiation at room temperature with X-rays is required to observe the bands. • EPR spectra show that the emitting centers are two distinct Ag²⁺ ions. • A charge-transfer mechanism is responsible for the luminescence. • X-ray-induced Ag⁰ centers have broad optical absorption bands in the ultraviolet