Abstract 3D representation of a crystalline structure consisting of several interconnected, transparent polyhedrons. The bodies are tetrahedral or octahedral in shape, with bright edges in yellow, blue, and orange.

Structure model of the newly discovered substance with extraordinary properties

Opti­cal tem­per­a­ture sens­ing from cold to very hot

An international collaboration involving researchers from the University of Innsbruck has developed a novel luminescent material that enables particularly robust and precise optical temperature sensing across an exceptionally broad temperature range.

Optical luminescence thermometry has been gaining increasing importance, as it allows contactless temperature measurement even under extreme conditions. A key concept in this field is so-called ratiometric Boltzmann thermometry, in which the intensity ratio of two thermally coupled emission transitions directly follows the temperature. The performance of such thermometers crucially depends on the electronic structure of the luminescent ion and its incorporation into the host structure.

In a recent study, the two first authors, Gülsüm Kinik from the research group of Prof. Markus Suta at Heinrich Heine University Düsseldorf and Ingo Widmann from the research group of Prof. Hubert Huppertz at the Department of General, Inorganic and Theoretical Chemistry at the University of Innsbruck, reported the compound Al0.993Cr0.007B4O6N, which stands out as an exceptionally high-performance luminescence thermometer. The material is based on Cr3+ ions embedded in an almost ideal octahedral coordination environment, resulting in a particularly well-defined energy level scheme.

This form of thermometry is based on the efficient thermal coupling of two excited states of the Cr3+ ions. The combination of very fast non-radiative coupling and comparatively slow radiative relaxation leads to a record-breaking dynamic working range extending from below 77 K (–196 °C) to above 873 K (600 °C). This significantly outperforms many established luminescent thermometers, including classical rare earth-based systems such as the standard example Er3+. At temperatures above 340 K (67 °C) an additional broadband emission can be utilized, enabling particularly sensitive thermometric measurements.

Grafische Darstellung eines physikalischen Zusammenhangs zwischen Intensität und Wellenlänge. Auf schwarzem Hintergrund sind zwei Kurven eingezeichnet: eine blaue und eine rote, die jeweils zwei spektrale Peaks zeigen.

Comprehensive experimental characterization, including structural analyses, temperature-dependent emission spectra, and kinetic investigations of relaxation processes in the excited states shows that Al0.993Cr0.007B4O6N is an exceptionally stable, red-emitting, and versatile thermometric material. Owing to its robustness, it is particularly well suited for applications under extreme conditions, such as high-pressure and high-temperature research.

The current results were published in the renowned journal Light: Science & Applications of the Nature Publishing Group and mark a significant step toward new, highly efficient luminescence thermometers based on transition-metal cations. The research groups are planning further work in this exciting field.

Publication: Gülsüm Kinik, Ingo Widmann, Benedikt Bendel, Hubert Huppertz, Andries Meijerink, Markus Suta: Ratiometric Boltzmann thermometry with Cr3+ in strong ligand fields: Efficient nonradiative coupling for record dynamic working ranges. Light: Sci. Appl. 2025,14, 388. DOI: 10.1038/s41377-025-02082-8

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