Near-zero thermal quenching of upconversion green emission implemented with the polymorphic phase transition

Thermally stable upconversion luminescence is highly expected for applications in high-temperature environments. Herein, the mechanism of the phase transition inducing near-zero thermal quenching in upconversion green light is reported. The phosphor Ba3Gd0.78Yb0.2Er0·02Al2O7.5 is synthesized by the conventional high-temperature solid-state method and quenched in air during cooling. In-situ XRD and DSC curves demonstrate the polymorphic phase transition in the subsequent heating. The temperature-dependent upconversion luminescence exhibits stable green emission instead of thermal quenching when the material is excited by a 980 nm near-infrared laser. The more intense green emission resulting from the low symmetry of the low-temperature phase and its increase in the phase content with temperature are responsible for the thermal stability in the luminescence of the material. This research provides an approach to utilizing phase transformation to obtain stable emissions in upconversion phosphors.