Temperature is a physical quantity that represents the degree of heat or cold of an object and has significant application value across various fields. Traditional contact temperature measurement technologies, such as thermocouples and infrared thermometers, suffer from limitations like poor environmental adaptability and low spatial resolution, which makes it difficult to meet the temperature measurement requirements for micro-/nano-devices and extreme environments. In recent years, non-contact optical temperature measurement technology based on the luminescence characteristics of rare earth ions has garnered widespread attention due to its high sensitivity, strong interference resistance, and good environmental adaptability. In addition to inorganic luminescent materials, lanthanide-based molecular and coordination-complex thermometers have also become an important branch of this field; however, this paper focuses on inorganic rare earth luminescence thermometry. This paper provides a systematic review of the mechanisms of temperature measurement using rare earth ion luminescence, including single-energy-level luminescence intensity measurement and luminescence intensity ratio measurement based on thermally coupled levels (TCLs) and non-thermally coupled levels (NTCLs). It analyzes the principles of various technologies, performance parameters (such as absolute sensitivity S_a, relative sensitivity S_r, and temperature resolution δT), and their application progress in fields such as biomedical imaging, high-temperature aerospace environments, and the integration of micro-/nano-devices. Special attention is paid to emerging research directions, including Stark sublevel engineering for enhanced sensitivity, negative thermal expansion (NTE) host design for anti-thermal quenching, multi-modal collaborative thermometry, and artificial intelligence (AI)-assisted material design and data processing. The article also discusses the challenges currently faced by the technology, such as high-temperature fluorescence quenching and signal interference, and looks forward to future development directions, including artificial intelligence-assisted material design and multi-modal cooperative temperature measurement, aiming to provide a reference for the research and application of rare earth luminescence temperature sensing technology.

Garnet-structured compounds have long been recognized as versatile host platforms for luminescent phosphors, owing to their robust chemical stability and highly tunable cationic sublattices. Conventional aluminate and gallate garnets, however, generally face intrinsic limitations, including severe concentration quenching at high dopant levels, relatively high phonon energy, and unsatisfactory efficiency for long-wavelength near-infrared (NIR) emission. In recent years, sodium-based germanate garnets, constructed by introducing aliovalent Na⁺ into dodecahedral sites combined with Ge⁴⁺ substitution in tetrahedral frameworks, have emerged as a promising branch of garnet phosphors. The aliovalent Na+ incorporation brings unique structural effects, including energy migration blocking, coordination environment distortion, and defect level modulation, which endow the materials with advantages in mitigating concentration quenching, boosting energy transfer efficiency, and enhancing thermal stability. This review systematically summarizes the crystal structure classification and luminescence fundamentals of sodium-based germanate garnet systems, and introduces mainstream synthesis techniques represented by the high-temperature solid-state method. Focusing on representative host systems, the luminescence characteristics and energy transfer mechanisms of both rare earth- and transition metal-doped systems are elaborated in detail, followed by a summary of four representative performance regulation strategies: cationic disorder engineering, crystal field engineering, defect engineering and dual-site cooperative regulation. The multifunctional applications of these materials in white light-emitting diodes, plant growth lighting, fluorescence temperature sensing, NIR imaging, and information encryption are also presented. Finally, existing challenges and future research perspectives are proposed to provide guidance for the development of high-performance garnet phosphors.

Red inorganic pigments traditionally rely on Pb-, Cd-, Hg-, or Cr(VI)-based colourants that carry avoidable hazard and exposure risks. Here, a family of Sr₆YAl_1−xFe_x(BO₃)₆ (0 ≤ x ≤ 1) pigments was prepared by solid-state reaction using Fe³⁺ as the chromophore and no Pb-, Cd-, Hg-, Cr(VI)-, or As-containing precursors. X-ray diffraction shows that the trigonal Sr₆YAl(BO₃)₆-type framework (space group R-3) is retained as a single phase throughout the series; on the basis of isovalent charge, six-coordinate geometry and matched ionic radii, Fe³⁺ substitutes for octahedral Al³⁺ to form a continuous solid solution, and SEM-EDS mapping confirms a broadly homogeneous, single-phase Fe-containing borate. Increasing Fe content strengthens visible absorption and moves the colour from white to a light reddish-orange (warm tan), with the x = 1 end member giving L* = 69.69, a* = 10.08, b* = 12.79 and C* = 16.28. The colour is essentially unchanged in water and dilute acetic acid, shows a moderate shift in alkaline media (ΔE* = 8.05–10.21), and changes markedly only in 0.1 mol L⁻¹ HCl (ΔE* = 15.07). Elemental release was screened by immersing 1.00 g of powder in 50 mL of water (H1), 0.01 mol L⁻¹ acetic acid (H2) or 0.1 mol L⁻¹ HCl (H3) for 24 h at room temperature, with each leachate read three times by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Water and dilute acid gave similar, modest release, whereas HCl mobilized substantial B, Sr and Y (530.72, 4639.42 and 470.54 mg L⁻¹, i.e., up to 41.87%, 45.15% and 27.08% of the nominal inventory) but very little Fe (0.156 mg L⁻¹, only ≈0.014% of the Fe inventory, and below detection in H1 and H2). The Fe chromophore is thus effectively immobilized even when the borate framework is attacked, decoupling colouration from chromophore leaching. The pigments demonstrate source-level elimination of regulated toxic metals together with strong chromophore retention; the non-standardized extraction provides a conservative release-potential screen rather than a regulatory compliance determination.