Upconversion enhancement and temperature sensing studies in Li⁺ ions incorporated GdPO₄:Tm³⁺/Yb³⁺ phosphor

The GdPO₄:Tm³⁺/Yb³⁺ phosphor codoped with various concentrations of Li ⁺ ions were synthesized for upconversion emission and optical thermometry studies. Excitation using 980 nm laser diode results in three upconversion (UC) emission bands with centre wavelengths of 478, 648 and 692 nm. These bands are originated from ¹G₄→³H₆, ¹G₄→³F₄, and ³F₃→³H₆ transitions of the Tm³⁺ ion, respectively. Li⁺ ions modified the local crystal symmetry around dopant ions, resulting enhanced UC emissions. The lifetime of the ¹G₄ level of Tm³⁺ ion was studied using a 980 nm laser excitation. The temperature sensing performances of GdPO₄:Tm³⁺/Yb³⁺ and GdPO₄:Tm³⁺/Yb³⁺/Li⁺ based on fluorescence intensity ratio (FIR) technique were evaluated in the temperature range 301-713 K under 980 nm excitation. Non-thermally coupled levels ³F₃ (692 nm) and ¹G₄ (478, 648 nm) were utilized for FIR estimation. A maximum absolute sensitivity of 6.28 × 10^-3 K^-1 at 653 K and 18.71 × 10^-3 K^-1 at 713 K were observed for Li ⁺ undoped and codoped phosphors respectively. The result indicates that codoping of Li ⁺ ions improved the UC emission as well as optical thermometry. Moreover, CIE colour coordinates and anti-counterfeiting application were also exhibited.