Beyond the energy gap law: the influence of selection rules and host compound effects on nonradiative transition rates in boltzmann thermometers

Apart from the energy gap law, control parameters over nonradiative transitions are so far only scarcely regarded. In this work, the impact of both covalence of the lanthanoid–ligand bond and varying bond distance on the magnitude of the intrinsic nonradiative decay rate between the excited ⁶P_5/2 and ⁶P_7/2 spin–orbit levels of Gd³⁺ is investigated in the chemically related compounds Y₂[B₂(SO₄)₆] and LaBO₃. Analysis of the temperature-dependent luminescence spectra reveals that the intrinsic nonradiative transition rates between the excited ⁶P_J (  J = 5/2, 7/2) levels are of the order of only 10 ms⁻¹ (Y₂[B₂(SO₄)₆]:Gd³⁺: 8.9 ms⁻¹; LaBO₃:Gd³⁺: 10.5 ms⁻¹) and differ due to the different degree of covalence of the Gd—O bonds in the two compounds. Comparison to the established luminescent Boltzmann thermometer Er³⁺ reveals, however, that the nonradiative transition rates between the excited levels of Gd³⁺ are over three orders of magnitude slower despite a similar energy gap and the presence of a single resonant phonon mode. This hints to a fundamental magnetic dipolar character of the nonradiative coupling in Gd³⁺. These findings can pave a way to control nonradiative transition rates and how to tune the dynamic range of luminescent Boltzmann thermometers.