Structure, spectroscopic properties and optical temperature-sensing behavior of glass-ceramics containing polymorphic CaTa2O6:Er3+/Yb3+ nanocrystals†

Novel glass-ceramics (GCs) containing polymorphic CaTa₂O₆:Er³⁺/Yb³⁺ nanocrystals were prepared in this study using an aerodynamic levitation method followed by heat treatment of the precursor glasses (PGs). The phase transition from cubic to orthorhombic crystal forms depending on the heat-treatment temperature was observed. The appearance of the corresponding GCs changed gradually from transparent to translucent and opaque with increasing temperatures. The evolution of phase composition and microstructure was investigated through X-ray diffraction (XRD), Rietveld refinement and transmission electron microscopy (TEM). Both XRD refinement results and spectroscopic properties, including the decrease in unit cell parameters, significant enhancement in upconversion (UC) luminescence, obvious Stark splitting of the UC and near-infrared (NIR) emission bands and the extension of lifetimes, confirmed the incorporation of the rare-earth (RE) ions into the crystalline phases. Furthermore, the temperature-dependent UC and NIR luminescence variations of the GCs containing polymorphic CaTa₂O₆:Er³⁺/Yb³⁺ nanocrystals were investigated. Based on the fluorescence intensity ratio (FIR) technique, the thermally coupled levels (TCLs) (²H_11/2/⁴S_3/2), non-thermally coupled levels (NTCLs) (⁴S_3/2/⁴F_9/2) and Stark sublevels (⁴I_13/2) of Er³⁺ ions were used for three-mode thermometry. The maximum absolute sensitivities of FIR(H/S) and FIR(S/F) were 3.8 × 10⁻³ and 3.7 × 10⁻² K⁻¹, respectively, which are much higher than those reported in previous reports. Furthermore, FIR(1500/1532) based on the Stark sublevels of orthorhombic phase GC offers a complementary way of temperature sensing. These results suggest the potential application of the GCs in self-referenced optical thermometry.