Thermochromic Upconversion Emission in Tm3+/Yb3+-Codoped La2Mo3O12 Microparticles via Negative Thermal Expansion Engineering for Ultrahigh Sensitivity Optical Thermometry

To develop high-sensitivity optical thermometers, Yb3+/Tm3+-codoped La2Mo3O12 microparticles were synthesized by the sol–gel method. With the aid of in situ X-ray diffraction, the resultant microparticles are verified to possess negative thermal expansion (NTE) properties. When excited at 980 nm, the upconversion (UC) emission properties of final products are investigated, in which their strongest fluorescence intensities are reached at x = 0.07. Due to the coexistence of the increased energy transfer, cross-relaxation, and nonradiative relaxation procedures, the as-prepared microparticles present thermochromic UC emissions. Moreover, the intensity of UC emission arising from the 3F2,3 excited level at 583 K is 21 times higher than its starting value at 303 K, resulting in thermally enhanced luminescence in resultant microparticles. By employing the fluorescence intensity ratio technique to investigate the temperature-related intensities of UC emissions from 1G4 and 3F2,3 levels, the thermometric characterization of designed compounds is explored, where its highest absolute and relative sensitivities are 0.44 K–1 and 7.37% K–1, respectively. Furthermore, according to the temperature-related lifetimes of 1G4 and 3F2,3 levels of Tm3+, the relative sensitivities of developed microparticles are 0.36% and 0.23% K–1, respectively. Ultimately, visual optical thermometry is also realized by the studied samples owing to their thermochromic UC emissions. Our findings propose a facile strategy by employing NTE to regulate the UC emission behaviors of rare-earth ions so as to obtain high-sensitive luminescent materials.