Temperature and pressure sensing properties based on FIR method modified by multiphoton processes in lead-free halide double perovskites

Lead-free halide double perovskites have emerged as promising candidates for optical sensing owing to their excellent structural stability and tunable optoelectronic properties. In this work, a series of Cs 2 Na 1− x Ag x InCl 6 :10 mol%Yb 3+ , 2 mol%Er 3+ ( x = 0–0.3) phosphors were synthesized to investigate the effects of Ag substitution on the crystal structure, upconversion luminescence, temperature- and pressure-sensing performances. X-ray diffraction and Raman scattering analyses reveal that Ag incorporation induces lattice contraction and shifts the Raman bands toward higher wavenumbers, suggesting enhanced local crystal-field interactions and modified phonon coupling. The upconversion emission intensity, luminescence lifetime, and effective photon number exhibit a nonmonotonic dependence on the Ag content, with the sample at x = 0.2 exhibiting the optimal luminescence performance, which can be attributed to a favorable balance between radiative and nonradiative relaxation processes. To enhance sensing reliability under excitation-power fluctuations, a photon-number-corrected fluorescence intensity ratio (FIR) strategy is proposed by incorporating the effective photon numbers into the conventional FIR model. Compared with the conventional FIR approach, the photon-number-corrected FIR exhibits substantially improved robustness against excitation-power fluctuations while maintaining high sensing performance. The maximum relative temperature sensitivity based on the thermally coupled I′ 550nm /I′ 525nm channel reaches 11.32% K −1 , while a maximum relative pressure sensitivity of 18.75% GPa −1 is achieved over the pressure range of 0–14 GPa. These results demonstrate that Ag-regulated Cs 2 Na 1−x Ag x InCl 6 :Yb 3+ , Er 3+ double perovskites are promising multifunctional optical materials for highly sensitive and fluctuation-resistant temperature and pressure sensing applications.