Pressure-Induced Symmetry Breaking Enables Retainable Upconversion Photoluminescence Enhancement in Lanthanide Oxyfluoride

Pressure-induced performance enhancement of a given material is an emerging phenomenon, however, among the various known examples very few enhanced properties can retain to ambient conditions. Both new structural mechanism and new material systems are the key challenge to achieve recoverable properties via pressure treatment. Herein, an "Er-rich" erbium oxyfluoride ErOF is reported with retainable enhanced upconversion (UC) emission by high pressure treatment. The UC emission increases to more than twice the intensity of its pristine state, and followed by additional improvement to 5.1-fold increase by heat treatment at 100 degrees C. The site symmetry breaking of Er3+ ions along with the crystal phase transition from R3 over bar $\bar{3}$m to Pnma, revealed by X-ray diffraction, is believed to be the underlying reasons for the enhanced UC emission after release. Further, high-resolution transmission electron microscopy (HRTEM) study reveals that the defects relieved with heat treatment might account for the UC emission enhancement in the released sample. Time-resolved spectra and I-P curves are measured comparatively to further illustrate the intrinsic mechanism of UC processes. The findings open up a window to advance UC performance through structure optimization by pressure engineering, thus facilitating its potential applications under ambient conditions. The retainable enhancement of upconversion (UC) emission is achieved in the erbium oxyfluoride by local symmetry breaking with the pressure engineering. The integral intensity of UC emission of pressure-treated sample increases to more than twice the intensity of its pristine state, and further to 5.1-fold enhancement by defects relieving through heating at low temperature. image