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

Abstract A new ratiometric Boltzmann thermometry approach is presented for the narrow-line red-emitting bright phosphor Al 0.993 Cr 0.007 B 4 O 6 N. It relies on thermalization between the two excited states 2 E g ( 2 G) and 2 T 1 g ( 2 G) of Cr 3+ 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 2 E g ( 2 G) 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 Er 3+ . At temperatures above 340 K, also spectrally well-resolved broad-band emission due to the spin-allowed 4 T 2 g ( 4 F) → 4 A 2 g ( 4 F) 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 0.993 Cr 0.007 B 4 O 6 N is a particularly robust and bright red luminescent thermometer with a record-breaking dynamic working range for a luminescent transition metal ion.