Abstracts

Temperature is the most fundamental thermodynamic state variable, which demands meticulous monitoring across various fields. Its precise measurement holds increasing significance in cutting-edge futuristic technologies such as superconductors and quantum information processing via quantum computing. In these domains, temperature plays a critical role, with the most efficient devices operating close to or slightly above the liquid nitrogen temperature (77 K). Consequently, thermometers capable of measuring such temperatures with low uncertainty are highly attractive. Because of such demand, luminescence thermometry has emerged to fulfill this need, leveraging the thermal dependence of electronic transitions to enable contactless temperature measurement. Thus, we propose a luminescent thermometer with low uncertainty that is tailored for cryogenic temperatures (close to liquid nitrogen). This thermometer based on SrY₂O₄:Eu³⁺(2,4,6,8 at.%) phosphor was synthesized using the modified Pechini method. The procedure employed stoichiometric proportions of the metallic nitrate solutions, Sr(NO₃)₂, and RE(NO₃)₃ (RE = Y³⁺, Eu³⁺), citric acid, and D-sorbitol to form a polymeric resin. This resin underwent pre-calcination at 350 °C/3 h, followed by annealing the precursor charcoal at 1100 °C/5 h. Analysis of the diffraction patterns via X-ray Diffraction (XRD) confirmed compatibility with the orthorhombic SrY₂O₄ matrix as the majoritary phase, validating the success of the synthesis method. Examination of the emission spectra at low (10 K) and room (298 K) temperatures revealed that the 4 at.% sample excelled among the others due to its higher relative intensity values. Interestingly, the ⁵D₀→⁷F₀ band split into two components, implying that at least two sites with C_n, C_nv, and/or C_s symmetry were replaced by Eu³⁺. Consequently, several components were observed for the most intense ⁵D₀→⁷F₂ band. This band exhibited a nonlinear pattern with temperature, as one of the components, namely Γ₁, gained intensity at higher temperatures, while the other component (Γ₂) reduced. This observation enabled us to use the ratiometric approach to construct a luminescent thermometer. The maximal relative sensitivity (S_r) reached values of 0.25% K⁻¹ in the 70 – 120 K temperature range. Despite the reasonably low value of S_r, one can have an extremely sensible thermometer but poor thermal accuracy and resolution, which can mislead the readout. In this sense, in the same temperature range, the uncertainty (δ_T) assumed values of 0.12 – 0.32 K. These are reasonably low values, where the first is comparable to δ_T obtained from multiparametric or dimensionality reduction approaches. Hence, the low uncertainty in the measurement yields high thermal resolution, implying that the proposed thermometer has potential for measuring temperatures in the cryogenic realm.