Polymorphism of Erbium Oxyfluoride: Selective Synthesis, Crystal Structure, and Phase-Dependent Upconversion Luminescence

Phase-selective synthesis and structure switching behavior of a functional material are essential to enable comparative studies on the structure-property relationship. Here, we report a controllable fluorination route to phase-pure erbium oxyfluorides with orthorhombic (O-ErOF) and rhombohedral (R-ErOF) structures. This facile method adopts polytetrafluoroethylene (PTFE) as the fluoridizer, and the phase selectivity can be easily achieved at specific fluorination temperatures. The phase evolution and detailed crystal structures of erbium oxyfluoride were characterized by powder X-ray diffraction (PXRD) at various sintering temperatures and Rietveld refinements, respectively. An irreversible phase transition from O-ErOF to R-ErOF was observed under heating around 600 degrees C. The upconversion (UC) luminescence properties of R-ErOF and O-ErOF were studied comparatively by means of photoluminescence, P-I, and UC decay curves. Despite their similar components and crystal structure, R-ErOF exhibits stronger (more than 20 times) red UC emission than O-ErOF. The anomalous UC behavior of the two polymorphs of ErOF was associated with the energy transfer processes dependent on their crystal structure.