Investigation on Upconversion Luminescence and Optical Temperature Sensing Behavior for Ba₂Gd₂Si₄O₁₃:Yb³⁺-Er³⁺/Ho³⁺/Tm³⁺ Phosphors

To explore new phosphors for temperature sensing with high detection sensitivity, the Yb³⁺-Er³⁺/Ho³⁺/Tm³⁺ doped Ba₂Gd₂Si₄O₁₃ (BGS) was designed. Different strategies were introduced based on the upconversion (UC) luminescence. For BGS:0.2Yb³⁺,0.02Er³⁺, the fluorescence intensity ratio (FIR) of two green emissions of Er³⁺ shows a gradual enhancement with increasing temperature due to the thermally coupled levels. The piecewise expression of sensitivity was proposed in the temperature range of 293–553 K based on the Boltzmann distribution. For BGS:0.2Yb³⁺,0.01Ho³⁺, the FIR of the red to green emissions of Ho³⁺ changes with temperature, showing a linear relationship from 293 to 453 K. The absolute sensitivity was gained to be 0.0452 K^–1. For BGS:0.2Yb³⁺,0.02Tm³⁺, the high absolute and relative sensitivities were both achieved by employing the thermally coupled (³F₂,³F₃) and ³H₄ levels of Tm³⁺. The above study could have special reference to the development of new luminescent materials with high sensitivity.