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Abstract The design of effective optical systems featuring high thermal sensitivity able to discriminate ever smaller variations of temperature in noncontact mode is of critical importance to face the challenges brought by the modern‐day technological revolution. If from one hand, the ratiometric optical thermometers based on Boltzmann distribution are demonstrated to be characterized by a unique reliability, on the other hand, robust performances in different environments are highly desired for new applications such as in situ thermal sensing of catalytic reactions. Here, the crystal field experienced by Cr 3+ in Bi 2 Al 4 O 9 is investigated demonstrating the potential of this system as ratiometric self‐referencing thermal sensor being characterized by high relative sensitivity (1.24% K −1 at 290 K). The remarkable absolute sensitivity results in an exceptional low thermal resolution of ≈0.2 K, 15‐fold lower than for the conventional Nd 3+ ‐based thermometers used in biological applications. The comparison of the performances among different systems evidences the potential of Cr 3+ ‐based thermometers with thermal resolution even lower than the state of the art diamond. In addition, the pH dependence of the photoluminescence emission confirms a high stability also at extreme conditions of basic and acid environments and the aging effects are tested for one week.
Back et al. (Sun,) studied this question.