Analysis of the magnetic dipolar nonradiative decay of the 5D1 level of Eu3+ in single-crystalline huntite-type REAl3(BO3)4:Eu3+ (RE = Y, Gd, Lu)

Abstract

The intraconfigurational 4f⁶ ↔ 4f⁶ transitions of Eu³⁺ provide sensitive insights into local symmetry and ligand fields, making Eu³⁺ a versatile probe ion for structural elucidation and temperature-dependent optics. In particular, the dynamics of the ⁵D₁ level are of central importance for applications in ratiometric luminescence thermometry but are strongly influenced by the phonon energy of the host compound, selection rules and the average Eu–ligand distance. In Eu³⁺-activated huntite-type borates REAl₃(BO₃)₄ (RE = Y, Gd, Lu), there is an almost trigonal prismatic coordination with a local D₃ symmetry of the RE³⁺ ion, whose subtle distortions vary systematically from RE = Gd to RE = Y to RE = Lu. These structural changes are reflected in a slight elongation of the ⁵D₀ decay time and a decreasing asymmetry ratio R₂, indicating a reduced induced electric–dipole component according to Judd–Ofelt theory. Temperature-dependent photoluminescence and time-resolved measurements determine the intrinsic nonradiative coupling rate between the ⁵D₁ and ⁵D₀ levels to be k_nr(0) ≈ 55 ms⁻¹. As expected, this increases with a shorter Eu–O bond lengths, but in GdAl₃(BO₃)₄, it shows an additional contribution that cannot be explained structurally and is attributed to a local acceleration effect caused by the paramagnetism of the surrounding Gd³⁺ ions. The temperature dependence of the ⁵D₀ decay time also reveals a thermal coupling of the excited states and a high-temperature-induced quenching via the Eu³⁺ ← O²⁻ ligand-to-metal charge transfer (LMCT) state. These results provide a comprehensive understanding of the excited state dynamics of Eu³⁺ in huntite-type borate and, at the same time, illustrate the potential and limitations of these host lattices for Eu³⁺-based luminescence thermometry.