Bi-doped compounds recently became the subject of an extensive research due to their possible applications as scintillator and phosphor materials. The oxides co-doped with Bi³⁺ and trivalent rare-earth ions were proposed as prospective phosphors for white light-emitting diodes and quantum cutting down-converting materials applicable for enhancement of silicon solar cells. Luminescence characteristics of different Bi³⁺-doped materials were found to be strongly different and ascribed to electronic transitions from the excited levels of a Bi³⁺ ion to its ground state, charge-transfer transitions, Bi³⁺ dimers or clusters, radiative decay of Bi³⁺-related localized or trapped excitons, etc. In this review, we compare the characteristics of the Bi³⁺-related luminescence in various compounds; discuss the possible origin of the corresponding luminescence centers as well as the processes resulting in their luminescence; consider the phenomenological models proposed to describe the excited-state dynamics of the Bi³⁺-related centers and determine the structure and parameters of their relaxed excited states; address an influence of different interactions (e.g., spin-orbit, electron-phonon, hyperfine) as well as the Bi³⁺ ion charge and volume compensating defects on the luminescence characteristics. The Bi-related luminescence arising from lower charge states (namely, Bi²⁺, Bi⁺, Bi⁰) is also reviewed.

Luminescence methods for non-contact temperature monitoring have evolved through improvements of hardware and sensor materials. Future advances in this field rely on the development of multimodal sensing capabilities of temperature probes and extend the temperature range across which they operate. The family of Cr-doped oxides appears particularly promising and we review their luminescence characteristics in light of their application in non-contact measurements of temperature over the 5–300 K range. Multimodal sensing utilizes the intensity ratio of emission lines, their wavelength shift, and the scintillation decay time constant. We carried out systematic studies of the temperature-induced changes in the luminescence of the Cr³⁺-doped oxides Al₂O₃, Ga₂O₃, Y₃Al₅O₁₂, and YAlO₃. The mechanism responsible for the temperature-dependent luminescence characteristic is discussed in terms of relevant models. It is shown that the thermally-induced processes of particle exchange, governing the dynamics of Cr³⁺ ion excited state populations, require low activation energy. This then translates into tangible changes of a luminescence parameter with temperature. We compare different schemes of temperature sensing and demonstrate that Ga₂O₃-Cr is a promising material for non-contact measurements at cryogenic temperatures. A temperature resolution better than ±1 K can be achieved by monitoring the luminescence intensity ratio (40–140 K) and decay time constant (80–300 K range).

Zinc gallate (ZnGa₂O₄) spinel ceramics doped with Mn²⁺ ions was prepared by a solid-state reaction at 1200 °C in air. Manganese concentration was equal to 0.05 mol.% of MnO with respect to ZnO. Ceramics produced in this way show an efficient green emission at about 505 nm under UV or X-ray excitations, which is caused by Mn²⁺ ions. This green emission is observed also as a relatively long afterglow (visible to the naked eye in the dark for about one hour) after switching-off the X-ray excitation. Time profiles of the beginning of glow and afterglow have been studied together with thermally stimulated (TSL) and optically stimulated (OSL) luminescence. Experimental results demonstrate a presence of few types of shallow and deep traps responsible for the observed afterglow and TSL/OSL emission of the material. The possibility of pulsed optical stimulation and time-resolved OSL characteristics of ZnGa₂O₄: Mn²⁺ has been reported for the first time. The presented results suggest the ZnGa₂O₄: Mn²⁺ spinel as a promising material for further fundamental research and possibility of application as a green long-lasting phosphor or storage phosphor for TSL/OSL radiation dosimetry.

A 0.25% iron (Fe³⁺)-doped LiGaO₂ phosphor was synthesized by a high-temperature solid-state reaction method. The phosphor was characterized utilizing X-ray diffraction (XRD), scanning electron microscopy (SEM), high-pressure photoluminescence, and photoluminescence decay measurement techniques using diamond anvil cells (DACs). The powder X-ray analysis shows that the phosphor is a β polymorph of LiGaO₂ with an orthorhombic crystallographic structure at room temperature. The SEM result also confirms the presence of well-dispersed micro-rod-like structures throughout the sample. The photoluminescence studies in the near-infrared (NIR) range were performed at ambient, low-temperature, and high-pressure conditions. The synthesized phosphor exhibits a photoluminescence band around 746 nm related to the ⁴T₁ → ⁶A₁ transition with a 28% quantum efficiency at ambient conditions, which shifts toward longer wavelengths with the increase of pressure. The excitation spectra of Fe³⁺ are very well fitted with the Tanabe-Sugano crystal-field theory. The phosphor luminescence decays with a millisecond lifetime. The high-pressure application transforms the β polymorph of LiGaO₂ into a trigonal α structure at the pressure of about 3 GPa. Further increase of pressure quenches the Fe³⁺ luminescence due to the amorphization process of the material. The prepared phosphor exhibits also mechanoluminescence properties in the NIR spectral region.

Detailed investigations of the spectroscopic properties of Cr ³⁺ ions in β-Ga ₂ O ₃ :0.05% Cr ³⁺ single crystals grown by the floating zone technique have been performed in the temperature range 4.5–550 K. The luminescence of Cr ³⁺ ions at low temperatures is due to narrow R-lines ( ² E → ⁴ A ₂ transitions) and their vibronic sidebands, whereas the broad intense band ( ⁴ T ₂ → ⁴ A ₂ transition) dominates at temperatures above 200 K. The vibronic sidebands of R-lines with amplitude of less than 5% of the R ₁ -line at a temperature of 4.5 K cover the region of 700–735 nm. The temperature dependences of the luminescence intensity and the decay time of Cr ³⁺ ions indicate the same mechanism of quenching of R-line intensity and shortening of lifetime for ² E of Cr ³⁺ ions. The temperature dependence of the R ₁ -line decay time of β-Ga ₂ O ₃ :Cr ³⁺ with maximal temperature coefficient |Δτ/Δ T | = 0.023 ms K ⁻¹ at 120 K and maximal specific sensivitity |(Δτ/Δ T )τ ⁻¹ | = 0.017 K ⁻¹ at 160 K indicates an application potential of this phosphor for low-temperature fluorescence thermometry.