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

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 Pnm2_i, Z=2. Thermally Stimulated Luminescence (TSL) investigations on the gamma irradiated sample showed an intense glow peak at 407 K and a weak glow peak at 500 K. The trap parameters namely activation energy (E), order of kinetics (b) and the frequency factor (s) for the main peak at 407 K were determined using the glow curve method. Fluorescence studies showed weak emission at ca 367 nm, which was attributed to f-d transition (4f⁶5d-4f⁷) of Eu²⁺ ions in the host lattice, whereas an intense emission in the range 570–650 nm was assigned to ⁵D₀→⁷F_J, 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 300–475 K 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 407 K. - Highlights: • Synthesis of SrB₄O₇ doped with Eu. • Fluorescence investigations to identify both di and trivalent europium species in the system. • Electron paramagnetic resonance studied on both the pristine and gamma irradiated samples to identify the paramagnetic centers in the system. • TSL studies to investigate the trap parameters. • EPR–TSL correlation to understand the luminescence mechanism