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 T-4(2) -> (4)A(2)/E-2 -> (4)A(2) emission ratio of Cr3+. On the basis of structural information of the local HfO6 octahedral site we demonstrated the potential of the CaHfO3:Cr3+ system by combining deep theoretical and experimental investigation. The material exhibits simultaneous emission from both the E-2 and T-4(2) excited states, following the Boltzmann law in a cryogenic temperature range of 40-150 K. The promising thermometric performance corroborates the potential of CaHfO3:Cr3+ as a Boltzmann cryothermometer, being characterized by a high relative sensitivity (similar to 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 T-4(2)-E-2 energy gap controlled by the crystal field of the local octahedral site, the design proposed herein could be expanded to develop new Cr3+-doped cryogenic thermometers.