Stark-splitting-assisted multi-ratio upconversion thermometry in Er³⁺-doped K₂YbF₅ phosphors

Orthorhombic phase K₂YbF₅: Er³⁺ phosphors were prepared through a hydrothermal route and investigated as Stark splitting upconversion thermometers. Under 980 nm excitation, the prepared phosphors exhibit characteristic green and red upconversion emissions of Er³⁺ ions, and power-dependent measurements indicate that the visible upconversion emissions are mainly governed by two-photon processes. The red emission band can be divided into four Stark splitting components. Based on the thermally induced quenching behaviors of the four Stark splitting components, a Stark-splitting-assisted multi-ratio thermometry strategy is proposed. For the optimized K₂YbF₅: 3% Er³⁺ phosphors, the maximum relative sensitivity reaches 8.78% K⁻¹ at 298 K, about 4.47 times that of the conventional ²H_11/2/⁴F_9/2 non-thermally coupled level thermometry strategy. Temperature cycling, uncertainty analysis, and measurements using a K₂YbF₅: 3% Er³⁺ composite film further confirm the repeatability and practical feasibility of the proposed thermometric strategy. These results suggest that Er³⁺-doped K₂YbF₅ is a promising fluoride platform for high-sensitivity upconversion optical thermometry and flexible temperature-sensing devices.