Ratiometric Boltzmann thermometry with Cr3+ in strong ligand fields: Efficient nonradiative coupling for record dynamic working ranges

A new ratiometric Boltzmann thermometry approach is presented for the narrow-line red-emitting bright phosphor Al_0.993Cr_0.007B₄O₆N. It relies on thermalization between the two excited states ²E_g(²G) and ²T_1g(²G) of Cr³⁺ with an energy gap of 620 cm^-1 for optimized thermometry at room temperature. It is shown that nonradiative coupling between these excited states is very fast, with rates in the order of several µs^-1. Due to the comparably slow radiative decay (k_r = 0.033 ms^-¹) of the lowest excited ²E_g(²G) state, the dynamic working range of this Boltzmann thermometer for the deep red spectral range is exceptionally wide, between <77 K and >873 K, even outperforming the classic workhorse example of Er³⁺. At temperatures above 340 K, also spectrally well-resolved broad-band emission due to the spin-allowed ⁴T_2g(⁴F) → ⁴A_2g(⁴F) transition is detectable, which simultaneously offers a possibility of very sensitive (S_r(500 K) > 2% K^-1) ratiometric Boltzmann-type crossover thermometry for higher temperatures. These findings imply that Al_0.993Cr_0.007B₄O₆N is a particularly robust and bright red luminescent thermometer with a record-breaking dynamic working range for a luminescent transition metal ion.