Impacts of 5d electron binding energy and electron-phonon coupling on luminescence of Ce³⁺ in Li₆Y(BO₃)₃

In this work, the crystal structure and electronic structure as well as the synchrotron radiation vacuum ultraviolet-ultraviolet-visible (VUV-UV-vis) luminescence properties of Li₆Y(BO₃)₃ (LYBO):Ce³⁺ phosphors were investigated in detail. The Rietveld refinement and DFT calculation reveal the P2₁/c monoclinic crystal phase and the direct band gap of the LYBO compound, respectively. Only one kind of Ce³⁺ 4f-5d transition is resolved in terms of the low temperature VUV-UV excitation, UV-vis emission spectra and luminescence decay curves. Furthermore, by constructing the vacuum referred binding energy (VRBE) scheme and applying the frequency-degenerate vibrational model, the impacts of 5d electron binding energy and electron-phonon coupling on luminescence of Ce³⁺ in LYBO are analysed. The results show that the Ce³⁺ emission in LYBO possesses a moderate intrinsic thermal stability. With the increase in concentration, the thermal stability of the emission gets worse due to the possible thermally-activated concentration quenching. In addition, the simulation of Ce³⁺ emission profile at low temperature reveals that the 4f-5d electronic transitions of Ce³⁺ ions can be treated to couple with one frequency-degenerate vibrational mode having the effective phonon energy of ∼257 cm⁻¹ with the corresponding Huang-Rhys parameter of ∼6, which indicates a strong electron-phonon interaction of Ce³⁺ luminescence in the Li₆Y(BO₃)₃ host. Finally, the X-ray excited luminescence spectrum of the LYBO:5%Ce³⁺ phosphor is measured to check the potential scintillator applications.