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.

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Luminescent materials with high thermal stability and quantum efficiency are extensively desired for indoor illumination. In this research, a series of Eu3+-activated KGd2F7 red-emitting nanoparticles were prepared at room temperature and their phase structure, morphology, luminescence properties, as well as thermal stability, have been studied in detail. Excited by 393 nm, the resultant nanoparticles emitted bright red emissions and its optimal status was realized when the Eu3+ content was 30 mol%, in which the concentration quenching mechanism was triggered by electric dipole–dipole interaction. Through theoretical analysis via the Judd–Ofelt theory, one knows that Eu3+ situates at the high symmetry sites in as-prepared nanoparticles. Moreover, the internal and extra quantum efficiencies of designed nanoparticles were dependent on Eu3+ content. Furthermore, the studied nanoparticles also had splendid thermal stability and the corresponding activation energy was 0.18 eV. Additionally, via employing the designed nanoparticles as red-emitting constituents, a warm white light-emitting diode (white-LED), which exhibits low correlated color temperature (4456 K), proper luminous efficiency (17.2 lm/W) and high color rendering index (88.3), was developed. Our findings illustrate that Eu3+-activated KGd2F7 nanoparticles with bright red emissions are able to be used to promote the performance of white-LED.

To settle the challenging of phosphors with dissatisfactory luminescence efficiency and serious thermal quenching, Eu2+-activated Sr8Si4O12Cl8 cyan-emitting phosphors were designed. Excited by 387 nm, dazzling cyan emission located at 492...