Luminescence spectra and energy transfer of Tm<sup>3+</sup> and Tb<sup>3+</sup> doped in LiMgPO<sub>4</sub> phosphors

Abstract

<sec> LiMgPO<sub>4</sub> (LMP) phosphor doped with rare earth is a promising radiation dosimeter material. Thermoluminescence (TL) spectroscopy is an effective method to study the carrier traps and luminescence centers in materials. To study the luminescent mechanism of rare-earth-doped LMP phosphors, LMP phosphors doped with Tm and Tb (LMP:Tm, LMP:Tb and LMP:Tm, Tb) are prepared by high temperature solid state reaction. The TL glow curve and TL spectrum of the phosphors are measured by the linear heating method, and compared with the photoluminescence (PL) spectrum. </sec><sec> The shape of the TL glow curve of LMP phosphor varies with the doping active impurities, but the TL glow curves can be fitted by seven TL peaks. Double-doping Tm<sup>3+</sup> and Tb<sup>3+</sup> in LMP phosphor can enhance the TL intensity of the fifth peak (<i>E</i> ~ 1.39 eV) and weaken the seventh TL peak (<i>E</i> ~ 1.70 eV). </sec><sec> By comparing the PL spectra with TL spectra of the phosphors, it can be seen that the TL spectra are more complex than the PL spectra excited by ultraviolet light (λ = 352 nm). In the TL process, the electrons which are excited to the conduction band release the energy to recombination centers, and the released energy can more effectively excite the rare earth ions from the ground state to excited states, resulting in the more TL emitting peaks than in the the PL process. </sec><sec> Although Tm<sup>3+</sup> and Tb<sup>3+</sup> can be the luminescent centers in LMP phosphors when Tm<sup>3+</sup> and Tb<sup>3+</sup> are doped in LMP simultaneously, Tb<sup>3+</sup> ions are likely to act as sensitizers in LMP:Tm and LMP:Tb phosphor, and Tb<sup>3+</sup> ions transfer energy to the low-energy Tm<sup>3+</sup> ions, which makes the luminescent centers, Tm<sup>3+</sup> ions, excited to the high-energy state and then de-excited with emitting light. This result is also proved by the fact that the luminescence decay of Tb<sup>3+</sup> in phosphors increases with Tm<sup>3+</sup> concentration increasing. The energy transfer through non-radiative transition is more significant at higher temperature. </sec>