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 Al0.993Cr0.007B4O6N. It relies on thermalization between the two excited states 2Eg(2G) and 2T1g(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 (kr = 0.033 ms-1) of the lowest excited 2Eg(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 4T2g(4F) → 4A2g(4F) transition is detectable, which simultaneously offers a possibility of very sensitive (Sr(500 K) > 2% K-1) ratiometric Boltzmann-type crossover thermometry for higher temperatures. These findings imply that Al0.993Cr0.007B4O6N is a particularly robust and bright red luminescent thermometer with a record-breaking dynamic working range for a luminescent transition metal ion.