Winning color-tunable upconversion luminescence and high-sensitive optical thermometry in K₃Gd(PO₄)₂:Yb³⁺,Er³⁺,Tm³

A series of Yb³⁺, Er³⁺, Tm³⁺ co-doped K₃Gd(PO₄)₂ are prepared via the solid-state reaction method. Upon 980 nm excitation, the synthesized samples present color-tunable upconversion luminescence ranging from yellow to blue with the increment of Tm³⁺ doping contents. The upconversion mechanisms of Yb³⁺, Er³⁺, Tm³⁺ co-doped system are systematically investigated in detail. Varying contents of Tm³⁺ can appropriately alter the upconversion emissions of blue, green and red via possible energy transfer processes. Furthermore, the thermometric performances of phosphors associated with upconversion luminescence are analyzed in the temperature region of 300-675 K. By employing non-thermally coupled energy levels (²H_11/2/⁴F_9/2 of Er³⁺), the maximum relative and absolute sensitivity reaches 0.78 % K^-1 and 0.168 K^-1. Combining the sensitivity characteristic and repeatability of thermometer, the luminescence intensity ratio technology based on non-thermally coupled energy levels may be a more effective choice for optical thermometry. These excellent results will pave an avenue to K₃Gd(PO₄)₂:Yb³⁺,Er³⁺,Tm³⁺ phosphors for the fields of color-tunable luminescence and non-contact temperature sensing.