Analysis of the magnetic dipolar nonradiative decay of the5D1level of Eu3+in single-crystalline huntite-type REAl3(BO3)4:Eu3+(RE = Y, Gd, Lu)

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 knr(0) approximate to 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.