Mixed-anion coordination around Eu³⁺ ions affects the luminescence properties related to the 4f–4f and charge transfer transitions.

The objective of this work is to review and assess the potential of $\mathrm{Mg}{\mathrm{B}}_4{\mathrm{O}}_7$:Ce,Li to fill in the gaps where the need for a new material for optically stimulated luminescence (OSL) dosimetry has been identified. We offer a critical assessment of the operational properties of $\mathrm{Mg}{\mathrm{B}}_4{\mathrm{O}}_7$:Ce,Li for OSL dosimetry, as reviewed in the literature and complemented by measurements of thermoluminescence spectroscopy, sensitivity, thermal stability, lifetime of the luminescence emission, dose response at high doses (>1000 Gy), fading and bleachability. Overall, compared with ${\mathrm{Al}}_2{\mathrm{O}}_3:\mathrm{C}$, for example, $\mathrm{Mg}{\mathrm{B}}_4{\mathrm{O}}_7$:Ce,Li shows a comparable OSL signal intensity following exposure to ionizing radiation, a higher saturation limit (ca 7000 Gy) and a shorter luminescence lifetime (31.5 ns). $\mathrm{Mg}{\mathrm{B}}_4{\mathrm{O}}_7$:Ce,Li is, however, not yet an optimum material for OSL dosimetry, as it exhibits anomalous fading and shallow traps. Further optimization is therefore needed, and possible avenues of investigation encompass gaining a better understanding of the roles of the synthesis route and dopants and of the nature of defects.

In Pr ³⁺ -doped Y ₃ Al ₅ O ₁₂ , the blue-to-UV upconversion luminescence is significantly more affected by concentration quenching than the UV Stokes luminescence, which is due to cross-relaxation involving the upconversion intermediate states ³ P _J and ¹ D ₂ .

The photoluminescence spectra and luminescence lifetimes of Eu³⁺-doped oxyhalides, YOX:Eu³⁺ (X = Cl or Br), and their temperature dependence on different halide species of mixed-anion coordinations were investigated and analyzed. In terms of the ionic and covalent nature of bonding, Eu³⁺ ions form different coordination polyhedra in the isostructural YOCl and YOBr hosts: a ninefold [Eu³⁺O₄Cl₅] and an eightfold [Eu³⁺O₄Br₄] polyhedra. The Judd–Ofelt Ω₂ parameter for YOCl:Eu³⁺ takes a very large value (=8.81 × 10⁻²⁰cm²) due to the ninefold polyhedron with C_4v symmetry. On the other hand, despite the same C_4v symmetry, YOBr:Eu³⁺ shows a very small Ω₂ parameter (=2.72 × 10⁻²⁰cm²) because of its structural similarity to the square antiprism polyhedron with D₄ symmetry. The Ω₄ parameters for YOX:Eu³⁺ are much larger than those for other Eu³⁺-doped oxides, possibly related to the covalency of halide anions, Cl⁻ and Br⁻, showing an intense luminescence band (⁵D₀→ ⁷F₄) at around 700 nm. The Eu³⁺ ions in these YOX hosts were excitable by charge transfer bands in 270–280 nm regions. The relaxation pathways from the charge transfer states to the initial states for luminescence are discussed, using the configuration coordinate diagrams with the spectroscopic characterization.