Effect of Cd²⁺ and K⁺ Co-Doping on Upconversion Emission in Y₂O₃:Yb³⁺/Er³⁺ Phosphors and Application in Optical Thermometry

The Cd²⁺ and K⁺ co-doped Y₂O₃:Yb³⁺/Er³⁺ upconversion phosphors were synthesized via the combustion route and analyzed for their crystal structure, spectroscopic properties, upconversion behavior, and optical thermometric capabilities. Powder X-ray diffraction analysis confirmed that the crystal structure of host remained intact, with varying lattice parameters observed: decreasing for lower concentrations of Cd²⁺ and K⁺ (1 mol %) and slightly increasing at 3 and 5 mol % doping. Scanning electron microscopy reveals the fine particulates of sizes around 100 nm in 5 mol % Cd²⁺ and K⁺ co-doped Y₂O₃:Yb³⁺/Er³⁺while without Cd²⁺ and K⁺co-doping, it shows agglomerated form with spherical and rod shaped particles.Spectroscopic analysis across the UV-visible-NIR spectral range (200–2000 nm) indicated that Cd²⁺ and K⁺ dopants did not alter the spectral characteristics of prepared upconversion phosphor Y₂O₃:Yb³⁺/Er³⁺. Under excitation of 980 and 932 nm diode lasers, the upconversion emission exhibited green and red bands attributed to Er³⁺ ions. The intensity of the red band was predominant under 980 nm excitation, whereas the green band showed higher emission intensities under 932 nm excitation. Cd²⁺ and K⁺ doping significantly influenced the emission intensities of both bands but did not affect the emission wavelengths, with intensities decreasing as dopant concentrations increased. The temperature-dependent upconversion emission intensity variation was studied for optical thermometric applications, revealing enhanced relative sensitivity in Cd²⁺ and K⁺ doped samples compared to undoped phosphor. The maximum relative sensitivity of 0.01455 K^–1 at 303.15 K has been achieved with the 5 mol % Cd²⁺ and K⁺ co-doped upconversion phosphor, suggesting its potential for optical thermometry. Furthermore, evaluation of CIE color coordinates derived from upconversion emission spectra showed that Cd²⁺ and K⁺ doping had negligible impact on color chromaticity.