High-sensitivity luminescent temperature sensors: MFX:1%Sm²⁺ (M = Sr, Ba, X = Cl, Br)

The use of lanthanide luminescence has advanced the field of remote temperature sensing. Luminescence intensity ratio methods relying on emission from two thermally coupled energy levels are popular but suffer from a limited temperature range. Here, we present a versatile luminescent thermometer: Ba(Sr)FBr(Cl):Sm²⁺. The Sm²⁺ ion benefits from multiple thermally coupled excited states to extend the temperature range and has strong parity-allowed 4f⁶→4f⁵5d¹ absorption to increase brightness. We conduct a comparative analysis of the temperature sensing performance of Sm²⁺ in BaFBr, BaFCl, SrFBr, and SrFCl and address the role of concentration, host, and Boltzmann equilibration. Different thermal coupling schemes, ⁵D₁-⁵D₀ and 4f⁵5d¹-⁵D₀, and temperature-dependent lifetimes enable accurate sensing between 350 and 800 kelvin. Differences in 4f⁵5d¹-⁵D₀ energy gap allows optimization for a temperature range of interest. This type of Sm²⁺-based thermometer holds great potential for temperature monitoring in the wide and relevant range up to 500°C.