Delayed Concentration Quenching of Luminescence Caused by Eu³⁺-Induced Phase Transition in LaSc₃(BO₃)₄

Abstract

Phase transitions induced by activator concentration should be avoided in most cases because they usually degrade luminescent properties of inorganic phosphors. However, in the LaSc3(BO3)4:xEu3+ (LSBO:xEu3+) phosphors investigated here, a delayed concentration quenching of luminescence caused by Eu3+-induced phase transition from the monoclinic α-LSBO to the trigonal β-LSBO is proved via XRD refinements, photoluminescent spectra, fluorescence decay curves, and Raman spectra. Increasing the doping concentration of Eu3+ within x lower than 0.8 benefits the transition from the α- to β-phase, which increases the fluorescent lifetime and reduces the nonradiative transition of Eu3+, thus compensating the Eu3+ luminescence and delaying the quenching point to a higher concentration. The inversion symmetry of the Eu3+ site in β-LSBO is demonstrated to be higher than that in α-LSBO by the Judd–Ofelt theory and a new model proposed to calculate the distance index representing the symmetry. Moreover, both the high quantum efficiency of 89% and the good thermal stability of the emission intensity at 400 K being 88% of that at 300 K make the optimal LSBO:0.8Eu3+ phosphor a promising application prospect. This kind of activator-induced phase transitions contributing to the enhanced luminescence and the delayed quenching concentration may provide a new and effective way for improving luminescent properties of phosphors.