Multicolor tuning and temperature-triggered anomalous Eu³⁺-related photoemission enhancement via interplay of accelerated energy transfer and release of defect-trapped electrons in the Tb³⁺,Eu³⁺-doped strontium–aluminum chlorites

Abstract

So far, a large number of rare earth (RE) and non-RE-doped emission-tunable crystals based on controllable energy transfer have become available, but numerous mechanistic issues, particularly for those that involve temperature-dependent energy transfer between the well-shielded 4f RE ions, lack comprehensive theoretical and experimental investigation, limiting greatly their development and applications in the future. Here, we design and report a type of Tb³⁺,Eu³⁺-doped Sr₃Al₂O₅Cl₂ phosphors capable of multiemissions upon excitation at 376 nm, through using the orthorhombic Sr₃Al₂O₅Cl₂ as the host lattice while the well-shielded 4f Tb³⁺ and Eu³⁺ ions as dual luminescent centers. Our results reveal that the energy transfer from Tb³⁺ to Eu³⁺ ions, happening via an electric dipole-quadrupole (d-q) interaction, can be controlled by the doping ratio of Tb³⁺ and Eu³⁺, leading to the tunable emissions from green (0.3159, 0.5572) to red (0.6579, 0.3046). It is found from time-resolved photoluminescence (PL) spectra that this energy transfer begins at t = 5 μs and gradually ends at t ≥ 200 μs. Moreover, from temperature-dependent PL results, we reveal that the Eu³⁺ emission features an anomalous intensity enhancement at the earlier heating state. With the density functional theory (DFT) calculations, we have screened the possibilities of site preferential substitution problem. By jointly taking into account the X-ray diffraction Rietveld refinement, DFT findings, and PL and thermoluminescence spectra, a mechanistic profile is proposed for illustrating the PL observations. In particular, our discussions reveal that the temperature-triggered Eu³⁺ emission enhancement is due to the interplay of the temperature-induced accelerated energy transfer and defect-trapped electrons that are released upon the thermal stimulation. Unlike most of reported phosphor materials that are always suggested for phosphor-converted white light-emitting diodes, we propose new application possibilities for Tb³⁺,Eu³⁺-doped Sr₃Al₂O₅Cl₂ phosphors, such as anticounterfeiting, temperature-controlled fluorescence sensor, data storage, and security devices.