Radiation induced centers in irradiated SrB4O7 doped europium and their role in thermally stimulated reactions: Thermally stimulated luminescence, fluorescence and electron paramagnetic resonance studies

Abstract

Polycrystalline samples of strontium tetraborate (SrB4O7: SBO) samples doped with different concentrations of europium (0%, 0.05%, 1% and 2%) were synthesized using solid state reaction in air at high temperature. These samples were characterized by X-ray diffraction (XRD), photoluminescence (PL), thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) techniques. X-ray diffraction studies indicated that SrB4O7 has orthorhombic structure with lattice parameters a=4.237±0.001, b=4.431±0.001, c=10.704±0.010, space group Pnm2i, Z=2. Thermally Stimulated Luminescence (TSL) investigations on the gamma irradiated sample showed an intense glow peak at 407K and a weak glow peak at 500K. The trap parameters namely activation energy (E), order of kinetics (b) and the frequency factor (s) for the main peak at 407K were determined using the glow curve method. Fluorescence studies showed weak emission at ca 367nm, which was attributed to f-d transition (4f65d-4f7) of Eu2+ ions in the host lattice, whereas an intense emission in the range 570–650nm was assigned to 5D0→7FJ, J=0,1,2,3 and 4 transition of Eu3+ in the host lattice. The large value of I(R)/I(O) suggested that Eu3+ ions are situated at sites without center of inversion (lower symmetry) in SBO matrix. EPR studies suggested the formation of boron–oxygen hole trapped center viz, BOHC, electron trapped at oxygen vacancies and some impurity radicals like NO32− and CO2− radicals in SBO on irradiation. EPR temperature variation studies in the range 300–475K on gamma irradiated europium doped samples, suggested a recombination process occurring between boronoxygen hole trapped species and electron trapped species and subsequently energy is transferred to Eu3+ ion, which on de-excitation yields thermally stimulated luminescence yielding an intense TSL glow peak at 407K.