Pushing the Limit of Boltzmann Distribution in Cr3+-Doped CaHfO3 for Cryogenic Thermometry

Luminescence Boltzmann thermometry is one of the most reliable techniques used to locally probe temperature in a contactless mode. However, to date, there is no report on cryogenic thermometers based on the highly sensitive and reliable Boltzmann-based 4 T 2 → 4 A 2 / 2 E → 4 A 2 emission ratio of Cr 3+ . On the basis of structural information of the local HfO 6 octahedral site we demonstrated the potential of the CaHfO 3:Cr 3+ system by combining deep theoretical and experimental investigation. The material exhibits simultaneous emission from both the 2 E and 4 T 2 excited states, following the Boltzmann law in a cryogenic temperature range of 40–150 K. The promising thermometric performance corroborates the potential of CaHfO 3:Cr 3+ as a Boltzmann cryothermometer, being characterized by a high relative sensitivity (∼ 2%·K –1 at 40 K) and exceptional thermal resolution (0.045–0.77 K in the 40–150 K range). Moreover, by exploiting the flexibility of the 4 T 2 - 2 E energy gap controlled by the crystal field of the local octahedral site, the design proposed herein could be expanded to develop new Cr 3+ -doped cryogenic thermometers.