Dual-mode optical thermometry based on up- and down-conversion photoluminescence in LiCaLa(MoO₄)₃:Er³⁺/Yb³⁺ phosphors...

The integration of down-conversion (DC) and up-conversion (UC) photoluminescence mechanisms has attracted significant attention for applications in optical thermometry and solid-state lighting. Combining both emission processes within a single material enables dual-mode temperature sensing, offering enhanced flexibility and precision. In this study, we report a pioneering investigation of the dual-mode thermometric performance of LiCaLa(MoO₄)₃ phosphors co-doped with Er³⁺ (0.02) and Yb³⁺ (0.15), synthesized via a solid-state reaction route. To the best of our knowledge, this is the first report demonstrating simultaneous DC and UC-based thermometric behavior in this host matrix. The structural and morphological features of the synthesized phosphors were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), while their optical properties were analyzed using photoluminescence (PL) spectroscopy. XRD patterns confirmed the formation of a pure monoclinic LiCaLa(MoO₄)₃ phase. Under UV excitation at 325 nm, green DC emissions from Er³⁺ ions were observed, whereas intense green UC luminescence was recorded under 980 nm near-infrared excitation. Co-doping with Yb³⁺ significantly enhanced both DC and UC emission intensities. A dual-mode optical thermometry approach was implemented using non-thermally coupled levels (NTCL) of Er³⁺, enabling simultaneous temperature evaluation from both DC and UC emissions. At 300 K, the relative sensitivities (S _r) reached 1.2% K^-1 for DC and 2.1% K^-1 for UC modes. At elevated temperatures (510 K), the maximum absolute sensitivities (S _a) were 13.6 × 10^-3 K^-1 (DC) and 25 × 10^-3 K^-1 (UC), respectively. The system demonstrated good temperature resolution, with uncertainties (δT) below 0.313 K, confirming its potential for precise and robust optical temperature sensing.