Comparative optical thermometry analysis using Na₂SrP₂O₇:Er³⁺/Yb³⁺ phosphors: evaluation of LIR_TCL and LIR_NTCL...

Optical thermometry is a valuable non-contact technique for temperature measurement, especially in environments where traditional methods are impractical. Despite its advantages, enhancing the precision of optical thermometers remains a significant challenge. In this study, we explored the thermometric properties of Na₂SrP₂O₇ phosphors co-doped with Er³⁺/Yb³⁺, synthesized via a solid-state reaction method, for temperature sensing within the 200-440 K range under 980 nm excitation. Upconversion (UC) luminescence, observed in the visible spectrum, was analyzed using the fluorescence intensity ratio (FIR) method, focusing on both thermally coupled (TCLs) and non-thermally coupled (NTCLs) energy levels of Er³⁺/Yb³⁺. Specifically, the transitions ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 were examined to calculate thermometric parameters. The maximum absolute sensitivity (S _A) and relative sensitivity (S _R) for the ²H_11/2 → ⁴I_15/2 to ⁴S_3/2 → ⁴I_15/2 transition were 0.0009 K^-1 and 0.6% K^-1, respectively, while for the ²H_11/2 → ⁴I_15/2 to ⁴F_9/2 → ⁴I_15/2 transition, S _A was 0.004 K^-1, with a maximum S _R of 1.14% K^-1. Furthermore, by employing a luminescence intensity ratio technique based on TCLs (LIR_TCL), the minimum temperature uncertainty (δT) was found to be 1.31 K at 320 K. In contrast, the luminescence intensity ratio method based on NTCLs (LIR_NTCL) yielded a much lower minimum δT value of 0.34 K at 200 K, indicating superior performance in terms of temperature resolution. These findings demonstrate that the LIR_NTCL technique provides more sensitive and accurate temperature measurement compared to LIR_TCL. The excellent temperature resolution and sensitivity of Na₂SrP₂O₇:Er³⁺/Yb³⁺ phosphors highlight their potential for highly accurate optical thermometry applications in scientific and industrial contexts.