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

Area of Science:

• Materials Science
• Spectroscopy
• Solid-State Chemistry

Background:

• Ratiometric Boltzmann thermometry leverages thermal population of excited states for temperature measurement.
• Developing novel phosphors with wide dynamic ranges is crucial for advanced thermometry.
• Chromium-doped materials are known for their luminescent properties and potential in sensing applications.

Purpose of the Study:

• To introduce a novel ratiometric Boltzmann thermometry approach using a specific Cr-doped borate phosphor.
• To characterize the thermometric properties of Al$_{0.993}$Cr$_{0.007}$B$_{4}$O$_{6}$N, focusing on its wide temperature sensing capabilities.
• To evaluate its performance against established thermometry standards.

Main Methods:

• Utilized ratiometric Boltzmann thermometry based on thermalization between Cr$^{3+}$ excited states (2Eg and 2T1g).
• Investigated the temperature-dependent luminescence spectra of Al$_{0.993}$Cr$_{0.007}$B$_{4}$O$_{6}$N across a broad temperature range.
• Analyzed nonradiative and radiative decay rates to understand thermometric performance.

Main Results:

• Achieved a wide dynamic working range for thermometry, from <77 K to >873 K, outperforming Er$^{3+}$.
• Demonstrated fast nonradiative coupling between excited states (µs$^{-1}$) and slow radiative decay (0.033 ms$^{-1}$).
• Observed spectrally resolved broad-band emission above 340 K, enabling sensitive crossover thermometry (Sr > 2% K$^{-1}$ at 500 K).

Conclusions:

• Al$_{0.993}$Cr$_{0.007}$B$_{4}$O$_{6}$N is a robust and bright red luminescent thermometer.
• The material exhibits a record-breaking dynamic working range for luminescent transition metal ions.
• This phosphor is suitable for sensitive ratiometric thermometry across a vast temperature spectrum.