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 Cr3+ in Bi2Al4O9 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 Nd3+‐based thermometers used in biological applications. The comparison of the performances among different systems evidences the potential of Cr3+‐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.

The inefficient luminescence performance of Ce3+ activated glasses is primarily responsible for their commercial failure compared to the Ce3+ activated crystalline materials that are widely used as phosphors and scintillators. We observed that this behavior is explicitly related to the intrinsic characteristics of the host material. Here, we present a systematic study on Ce3+ luminescence in amorphous borate glass and make a comparison with the well-known polycrystalline Y3Al5O12:Ce3+ (YAG:Ce) phosphor. In borate glass, Ce3+ exhibits blue luminescence with quantum yield (QY) of about 42%, whereas the QY is more than 85% in YAG:Ce ceramic that exhibits yellow luminescence. This typical behavior has been discussed in terms of the site rigidity of dopant ions in the glassy and crystalline hosts, and its influence on the Ce3+ 5d j states’ crystal field splitting, Stokes shift, and the centroid shift, as well as the probability of thermal ionization, host’s intrinsic absorption, and the influence of Ce4+ impurity presence in the respective host materials. This study gives a quantitative understanding of host’s contribution on dopant’s luminescence properties and thereby provides an optimization guideline, which is highly demanding for the design of novel luminescent materials.

After more than a century of studies on the optical properties of Bi 3+ ions, the assignment of the nature of the emissions and the bands of the absorption spectra remain ambiguous. Here, we report an insight into the spectroscopy of Bi 3+ -activated CaMO 3 perovskites (M = Zr, Sn, and Ti), discussing the factors driving the metal-to-metal charge transfer and sp → s 2 transitions. With the aim to figure out the whole scenario, a combined experimental and theoretical approach is employed. The comparison between the temperature dependence of the photoluminescence emissions with the temperature dependence of the exciton energy of the systems has led to an unprecedented evidence of the chargetransfer character of the emitting states in Bi 3+ -activated phosphors. Low-temperature vacuum ultraviolet spectroscopy together with the design of the vacuum-referred binding energy diagram of the luminescent center is exploited to shed light on the origin of the absorption bands. In addition, the X-ray absorption near the edge structure unambiguously confirmed the stabilization of Bi 3+ in the Ca 2+ site in both CaSnO 3 and CaZrO 3 perovskites. This breakthrough into the understanding of the excited-state origin of Bi 3+ could pave the way toward the design of a new generation of effective Bi 3+ -activated phosphors.

Bismuth-based fluoride nanocrystalline particles have recently attracted much attention as hosts for luminescent ions such as lanthanides (Ln) being proposed for lighting devices and biological applications. However, a comprehensive investigation on the chemical properties of this family of materials, the growth of the nanoparticles, and information about the chemical and thermal stabilities are critical to assess the real potential of nanosystems. In this view, a combined experimental and theoretical approach is employed to investigate the crystalline and electronic structure of BiF3 and NaBiF4. A detailed spectroscopic investigation allows us to measure the exciton peaks of these fluoride compounds for the first time and to design the vacuum referred binding energy level diagram of the lanthanide-doped fluorides with respect to the valence and conduction bands of the hosts in comparison with conventional fluorides. In addition, temperature and water effects on the chemical stability of NaBiF4 were addressed, evidencing detrimental limitations and envisaging possible solutions in view of biological applications.

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