Detailed spectroscopic analysis of the electronic configuration of Cr3+ in Bi2Ga4O9 is reported. The material exhibits unique luminescent properties arising from the crystal field experienced by Cr3+, with simultaneous strong sharp and broadband near-infrared emissions from the 2E and 4T2 excited states, in a wide range of temperature. The system displays dual near-infrared emission characterized by a remarkable thermal sensitivity over the whole explored range of temperatures, reaching a value of 0.7 %•K-1 in the physiological range. Moreover, the possibility to absorb and emit in the first biological window, allows to consider the system as a new promising candidate for ratiometric fluorescent thermal sensing in biotechnological applications

The increasing interest in the development of ratiometric optical thermal sensors has led to a wide variety of new systems with promising properties. Among them, singly-doped ratiometric thermometers were recently demonstrated to be particularly reliable. With the aim to discuss the development of an ideal optical thermal sensor, a combined experimental and theoretical insight into the spectroscopy of the Bi2Ga4O9:Cr3+ system is reported showing the importance of an insightful analysis in a wide temperature range. Low-temperature photoluminescence analysis (from 10 K) and the temperature dependence of the lifetime investigation, together with the crystal field analysis and the modeling of the thermal quenching process, allow the estimation of key parameters such as the Debye temperature (cutoff frequency), the Huang–Rhys parameter, and the energy barrier between 2Eg and 4T2g. Additionally, by considering the reliable class of singly-doped ratiometric thermometers based on a couple of excited states obeying the Boltzmann law, the important role played by the absolute sensitivity was discussed and the great potential of Cr3+ singly-activated systems was demonstrated. The results may provide new guidelines for the design of reliable optical thermometers with outstanding and robust performances.

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.