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

• Published in: Journal of luminescence 2014-09, Vol.153, p.79-84
• Main Authors: Brant, A.T., Buchanan, D.A., McClory, J.W., Adamiv, V.T., Burak, Ya.V., Halliburton, L.E., Giles, N.C.
• Format: Article
• Language: English
• Subjects: Borate compounds; Charge transfer; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Electron paramagnetic resonance and relaxation; EPR; Exact sciences and technology; Magnetic resonances and relaxations in condensed matter, mössbauer effect; Optical absorption; Optical constants: refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of bulk materials and thin films; Photoluminescence; Physical radiation effects, radiation damage; Physics; Structure of solids and liquids; crystallography; X ray effects; X-ray irradiation
• ISSN: 0022-2313; 1872-7883
• DOI: 10.1016/j.jlumin.2014.03.008

Two broad photoluminescence (PL) bands peaking near 502 and 725nm 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 325nm. 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 502nm emission to Center A and the 725nm 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. •Photoluminescence bands peaking near 502 and 725nm 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.