EPR and Luminescence of ${\hbox {F}}^{+}$ Centers in Bulk and Nanophosphor Oxyorthosilicates

The main thermally stimulated luminescence glow peak in irradiated oxyorthosilicates occurs near 360-400 K and has been postulated to comprise an electron trapped at an oxygen vacancy (F⁺ center). We have used electron paramagnetic resonance spectroscopy to identify this defect in Ln₂SiO₅:Ce (Ln = Lu and Y) and show that it consists of a single electron trapped at a non-silicon-bonded oxygen vacancy in both bulk and nanophosphor oxyorthosilicates. Both Lu- and Y-based nanophosphors form seven- and nine-oxygen coordinated structures (P2₁/c) whereas the bulk phosphors form six- and seven-oxygen coordinated structures (C2/c). In each case the F⁺ center predominately forms at the larger oxygen site. A typical resonance comprises a single Gaussian line broadened by hyperflne interaction with g-values near the free electron value and hyperflne coupling ~0.4 mT. The F⁺ center can be annealed and radiation-induced, consistent with the thermally stimulated luminescence glow peak behavior.