Intense green up/down conversion emission and high-sensitivity optical thermometry of NaYb(WO₄)₂:Er³⁺ stoichiometric phosphors

Optical thermometry based on fluorescence intensity ratio (FIR) has garnered significant interest among researchers due to its rapid, non-contact, and high sensitivity characteristics. In this study, stoichiometric NaYb(WO4)(2) phosphors, doped with varying concentrations of Er3+, were successfully synthesized using the high-temperature solid-state method. Under both 980 nm and 266 nm excitation, the samples exhibited strong green and very weak red upconversion (UC) and downconversion (DC) luminescence. The structural and morphological properties of the prepared samples were thoroughly characterized using techniques such as X-ray diffraction (XRD), XRD Rietveld refinement, scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectroscopy. The phosphors demonstrated high-purity UC and DC emission, with maximum green-to-red ratios of 60.9 and 77.0, respectively. This discovery opens up vast possibilities for their application as pure green color converters. The UC emission of Er3+ ions in the stoichiometric NaYb(WO4)(2) phosphor was identified as a two-photon UC process, as evidenced by the relationship between the luminescence intensity and the excitation power of the sample. Moreover, the temperature-dependent emission spectra of UC and DC were monitored, and investigated the fluorescence intensity ratios (FIRs) of H-2(11/2) -> I-4(15/2) and S-4(3/2) -> I-4(15/2) transitions of Er3+ ions to assess the optical temperature sensing behavior of NaYb(WO4)(2):Er3+. The experimental results revealed that the maximum absolute sensitivities (S-A) of NaYb(WO4)(2) phosphors were 0.0120 K-1 (UC) and 0.0077 K-1 (DC), and the maximum relative sensitivities (S-R) were 0.0089 K-1 (UC) and 0.0092 K-1 (DC), respectively. These findings highlight the promising of Er3+-doped Yb3+ stoichiometric NaYb(WO4)(2) phosphors in optical temperature sensors.