Robust Bi³⁺-Eu³⁺ Co-doped olivine-type phosphor for optical thermometry: High sensitivity and pH resistance

Recently, Bi³⁺-doped inorganic phosphors have garnered significant research interest due to their promising optoelectronic and thermometric applications. Herein, we present a novel and underexplored olivine type phosphor, CaLuGaO₄:Bi³⁺,Eu³⁺, which has been synthesized, thoroughly characterized and analyzed. The compound contains octahedrally coordinated Ca²⁺ and Lu³⁺ sites exhibiting significant crystal field splitting energy, which facilitates the incorporation of Bi³⁺ and Eu³⁺ to generate blue (³P₁ → ¹S₀) and red (⁵D₀ → ⁷F_J) emissions, respectively. The emission color tunability in Bi³⁺/Eu³⁺ codoped phosphors was realized through controlled variation of dopant concentrations and facilitating energy transfer between the ions. The ratio of Eu³⁺ to Bi³⁺ luminescence intensities displayed pronounced temperature dependence across the 298-473 K range, enabling the phosphor to function as a ratiometric optical temperature sensor. The phosphor demonstrated excellent thermometric characteristics, reaching 0.43 % K^-1 (absolute) and 1.07 % K^-1 (relative) sensitivity values. Furthermore, the phosphor exhibited highly stable and pH-independent optical properties in solutions (pH = 3, 7, and 12), highlighting its strong potential for optical thermometry applications.