Mn⁴⁺/Eu³⁺ co-doped fluoride toward a blue light-excited optical fiber thermometer

The ongoing development of ratiometric optical thermometry is mainly trapped in thermally coupled levels of rare-earth ions and inefficient ultraviolet excitation. Herein, a new-type multiple sharp line emitting, blue light-excited K₂NaInF₆:Mn⁴⁺, Eu³⁺ fluoride phosphor has been reported as a ratiometric thermometer. The f-f transition of Eu³⁺ paves a steady reference to a highly temperature sensitive Mn⁴⁺d-d transition and enables high relative sensitivity of 1.65% K^-1 at 573 K. An optical fiber thermometry on a household oven with a relative standard deviation of 0.11% surpasses the standard of precision measurement, showing great potential in practical application. This discovery offers a highly sensitive neotype blue light-excitable ratiometric temperature sensor, that is Mn⁴⁺-doped fluoride, promoting practical applications of optical thermometry.