EPR and TSL studies on MgB 4O 7 doped with Tm: role of BO 32− in TSL glow peak at 470 K

Abstract

Electron paramagnetic resonance (EPR), thermally stimulated luminescence (TSL), photoacoustic spectrometric (PAS) and fluorescence studies were conducted on gamma-irradiated MgB 4O 7: Tm phosphor. EPR spectra of irradiated samples revealed the formation of BO 3 2 - , 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 ∥ ≈ 14 G , A ⊥ = 7 G , respectively. In addition to this, the radical without any hyperfine structure was identified as O 2 - ( 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 BO 3 2 - radical around 470 K. The observed TSL emission is caused by the recombination of thermally released holes from BO 3 2 - radical with Tm 2 + ion. The energy released in the electron–hole recombination process is used for the excitation of Tm 3 + resulting in a prominent glow 470 K. TSL emission studies confirmed that Tm 3 + acts as luminescent center for the prominent peak at 470 K. The trap depth for this peak has been determined to be 0.975 ± 0.03 eV from both TSL and EPR methods.