Temperature effects on luminescence of YNbO₄:Er³⁺/Tm³⁺/Yb³⁺ white-light upconversion phosphor and sensing behavior based on thermally and non-thermally coupled levels

Tri-doped phosphor YNbO₄:Er³⁺/Tm³⁺/Yb³⁺ was prepared by solid-state reaction method. The XRD and SEM results reveal that the product is a monoclinic phase of YNbO₄ with high crystallinity and the particles presenting grains with well-defined boundaries with average size is about 1–5 μm. The temperature-dependent luminescence is investigated with 980 nm laser excitation, whereas temperature sensing behaviour was studied in the range of 300–500 K based on Stark sublevels ¹G_4(a)/¹G_4(b) (Tm³⁺), thermal coupling levels ²H_11/2/⁴S_3/2 (Er³⁺) and non-thermal coupling levels ⁴F_9/2 → ⁴I_15/2 (Er³⁺) /³F₂ → ³H₆ (Tm³⁺), ²H_11/2 → ⁴I_15/2 (Er³⁺) / ⁴F_9/2 → ⁴I_15/2 (Tm³⁺) and ⁴S_3/2 → ⁴I_15/2 (Er³⁺) /³F₂ → ³H₆ (Tm³⁺), utilizing fluorescence intensity ratio (FIR) technique. It is worth highlighting that ¹G_4(a)/¹G_4(b) ratio as is not much employed in thermometry. Regarding, NTCL ³F₂/¹G_4(b) pair exhibited the highest absolute sensitivity (S_A) is about 110.2 × 10^− 3 K^− 1 at 500 K, whereas NTCL ³F₂/⁴F_9/2 pair presented the highest relative sensitivity (S_R) is 2.1% K^− 1 at 339 K. Meanwhile, the temperature-dependent emission colours are discussed in detail. High sensitivity indicates that YNbO₄:Er³⁺/Tm³⁺/Yb³⁺ white-light phosphor can be used in non-contact temperature sensors applications.