High-frequency EPR investigation of X-ray storage SrBPO5:Eu phosphor

High-frequency EPR (HFEPR) spectra of the SrBPO5:Eu with stillwellite structure, which was prepared using EuCl3.6H2O as a dopant, have been examined in order to clarify the Eu sites in it. They showed an intense doublet at 3.3 T corresponding to g=2.0 as is expected for the 8S state ions. Also, weak satellites were observed on both sides of the central doublet signal symmetrically. This means that microwave energy at 90 GHz is sufficiently larger than the zero-field splitting. Taking into account the crystal structure of stillwellite and the earlier PL investigation, a single Eu2+ site was assumed for the HFEPR simulation. As a result, it was found that HFEPR spectrum simulated using g=1.98161, ∣D∣=0.130 T and λ=∣E/D∣=0.06 reasonably agrees well with the experimental one, supporting that the Eu2+ occupies the Sr site. The large ∣D∣ value small λ value obtained implies that the crystal field surrounding Eu2+ has a strong axial field with weak rhombic distortion.