Thermal Enhanced Near-Infrared Upconversion Luminescence\nin Y<sub>2</sub>Mo<sub>4</sub>O<sub>15</sub>:Yb/Nd with Uniaxial Negative\nThermal Expansion

Thermal\nquenching (TQ) of luminescence presents a significant barrier\nto the effective use of optical thermometers in high-temperature applications.\nHerein, we report a novel uniaxial negative thermal expansion (NTE)\nphosphor, Y<sub>2–2<i>x</i>–2<i>y</i></sub>Mo<sub>4</sub>O<sub>15</sub>:<i>x</i>Yb,<i>y</i>Nd, synthesized by a solid-state reaction. Under 980 nm laser excitation,\nit exhibits excellent thermally enhanced near-infrared (NIR) upconversion\nluminescence (UCL) performance. The UCL intensities of Nd<sup>3+</sup> at 573 K were enhanced by 396-fold (750 nm), 57.6-fold (810 nm),\nand 7.6-fold (882 nm), respectively, compared with that of room temperature.\nIn situ temperature-dependent X-ray diffraction and steady- and transient-state\nspectra are used to reveal thermal expansion behavior and luminescence\nmechanism in detail. The thermal enhancement of NIR UCL is attributed\nto the synergistic effect of increased radiative transition probability\ndue to the anisotropic thermal expansion of the crystal and the enhanced\nenergy transfer (ET) efficiency resulting from uniaxial shrinkage\nand the phonon-assisted process. Based on the luminescence intensity\nratio (LIR) of the thermally coupled energy levels (<sup>4</sup>F<sub>7/2</sub>/<sup>4</sup>F<sub>3/2</sub>), the target sample achieved\nultrahigh sensitivity (<i>S</i><sub>r</sub> = 3.0% K<sup>–1</sup> at 298 K) with high repeatability over the entire\ntemperature range. This study not only provides a fresh perspective\nfor achieving thermal enhancement of NIR UCL phosphors using uniaxial\nnegative thermal expansion materials but also presents a novel approach\nfor developing NIR UCL optical thermometers with outstanding temperature\nperformance.