Thermal enhancement of upconversion by negative lattice expansion in orthorhombic Yb${}_2$W${}_3$O${}_{12}$

Thermal quenching of photoluminescence represents a significant obstacle to practical applications such as lighting, display, and photovoltaics. Herein, a novel strategy is established to enhance upconversion luminescence at elevated temperatures based on the use of negative thermal expansion host materials. Lanthanide-doped orthorhombic Yb${}_2$W${}_3$O${}_{12}$ crystals are synthesized and characterized by in situ X-ray diffraction and photoluminescence spectroscopy. The thermally induced contraction and distortion of the host lattice is demonstrated to enhance the collection of excitation energy by activator ions. When the temperature is increased from 303 to 573 K, a 29-fold enhancement of green upconversion luminescence in Er${}^{3+}$ activators is achieved. Moreover, the temperature dependence of the upconversion luminescence is reversible. The thermally enhanced upconversion is developed as a sensitive ratiometric thermometer by referring to a thermally quenched upconversion. (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)