EPR and TSL studies on MgB4O7 doped with Tm: role of BO32− in TSL glow peak at 470 K

Electron paramagnetic resonance (EPR), thermally stimulated luminescence (TSL), photoacoustic spectrometric (PAS) and fluorescence studies were conducted on gamma-irradiated MgB4O7: Tm phosphor. EPR spectra of irradiated samples revealed the formation of BO32-, a hole-trapped radical at room temperature. The principal values for the g and A tensors are g∥≈2.0071, g⊥=2.0131 and A∥≈14G, A⊥=7G, respectively. In addition to this, the radical without any hyperfine structure was identified as O2-(g∥=2.0385 and g⊥=2.0023). TSL studies showed the presence of an intense glow peak around 470 K and a weak shoulder at 550 K. Temperature dependence studies of the EPR signals in the range 300-600 K showed the thermal destruction of BO32- radical around 470 K. The observed TSL emission is caused by the recombination of thermally released holes from BO32- radical with Tm2+ ion. The energy released in the electron-hole recombination process is used for the excitation of Tm3+ resulting in a prominent glow 470 K. TSL emission studies confirmed that Tm3+ acts as luminescent center for the prominent peak at 470 K. The trap depth for this peak has been determined to be 0.975±0.03eV from both TSL and EPR methods.