Thermal Enhanced Near-Infrared Upconversion Luminescence in Y₂Mo₄O₁₅:Yb/Nd with Uniaxial Negative Thermal Expansion

Thermal quenching (TQ) of luminescence presents a significant barrier to the effective use of optical thermometers in high-temperature applications. Herein, we report a novel uniaxial negative thermal expansion (NTE) phosphor, Y_{2-2<i>x</i>-2<i>y</i>}Mo₄O₁₅:<i>x</i>Yb,<i>y</i>Nd, synthesized by a solid-state reaction. Under 980 nm laser excitation, it exhibits excellent thermally enhanced near-infrared (NIR) upconversion luminescence (UCL) performance. The UCL intensities of Nd³⁺ at 573 K were enhanced by 396-fold (750 nm), 57.6-fold (810 nm), and 7.6-fold (882 nm), respectively, compared with that of room temperature. In situ temperature-dependent X-ray diffraction and steady- and transient-state spectra are used to reveal thermal expansion behavior and luminescence mechanism in detail. The thermal enhancement of NIR UCL is attributed to the synergistic effect of increased radiative transition probability due to the anisotropic thermal expansion of the crystal and the enhanced energy transfer (ET) efficiency resulting from uniaxial shrinkage and the phonon-assisted process. Based on the luminescence intensity ratio (LIR) of the thermally coupled energy levels (⁴F_7/2/⁴F_3/2), the target sample achieved ultrahigh sensitivity (<i>S</i>_r = 3.0% K^-1 at 298 K) with high repeatability over the entire temperature range. This study not only provides a fresh perspective for achieving thermal enhancement of NIR UCL phosphors using uniaxial negative thermal expansion materials but also presents a novel approach for developing NIR UCL optical thermometers with outstanding temperature performance.