Li3Ba2Gd3(WO4)8: Er/Yb quaternary tungstate for high-sensitivity optical temperature sensing

: Quaternary Li 3 Ba 2 Gd 3 (WO 4 ) 8 phosphors co-doped with Er 3+ and Yb 3+ ions were investigated for their structural, optical, and previously unexplored thermometric properties. X-ray diffraction confirmed a pure monoclinic C2/c phase, while FTIR and Raman spectroscopy verified the presence of [WO 4 ] 2- units. UV-Vis diffuse reflectance revealed Er 3+ absorption bands and an expanded bandgap (2.87- 3.16 eV), highlighting efficient rare-earth incorporation. Under 979 nm excitation, the phosphors displayed intense green upconversion emissions ( 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 ), accompanied by weaker red and NIR bands, arising from efficient Yb 3+ → Er 3+ energy transfer dominated by two-photon processes. Temperature-dependent studies revealed remarkable thermal stability, negligible spectral shifts, and highly repeatability luminescence under cyclic heating. The optimized composition (2% Er 3+ / 20% Yb 3+ ) exhibited outstanding thermometric performance, with relative sensitivities reaching 2.166 %·K -1 and low temperature uncertainty (δT = 0.025–0.042 K). Overall, the quaternary LBGW: Yb 3+ /Er 3+ phosphors demonstrate strong upconversion, remarkable thermal stability, and precise optical temperature sensing, making them highly suitable for advanced temperature-sensitive technologies.