The stability of coordination polyhedrons and distribution of europium ions in Ca6BaP4O17

In this work, the coordination polyhedron stabilities and distributions of europium ions in Ca6BaP4O17 (CBPO) luminescent materials are investigated. The density functional theory (DFT)-based first principles calculation results show that the PO4 tetrahedrons can tilt in the structure, which leads to the atomic distortion of O13 and O12 in CBPO and the Eu2+/Eu3+-doped systems. The energy scale of about ∼0.1 eV suggests that stabilities of coordination polyhedrons are easily influenced by dynamic factors. The atomic distortion and vacancy of work as charge compensations in CBPO:Eu3+, and three lattice sites of europium are extracted and summarized. The X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) confirm that Eu3+ can occupy the Ca1, Ca2 and Ba sites of CBPO. The combination of first principles calculation and X-ray absorption fine structure (XAFS) provides more information about microstructures of luminescent materials.

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.

The critical issues of Ce³⁺ luminescence including thermal stability and spectral profile are discussed in this work.