Selective synthesis, polymorphism, reversible phase transition and structure-dependent optical functionalities of gadolinium oxyfluorides

Abstract

Polymorphism in functional materials offers an excellent platform for comparative investigations of the structure–property relationship and thus the rational design of functional materials with enhanced performances. Here, we report a facile and selective fluorination route to phase-pure gadolinium oxyfluorides crystallizing in orthorhombic (O-GdOF) and rhombohedral (R-GdOF) structures. The phase selectivity can be achieved simply by controlling the reaction temperature and the amount of the fluoridizer (polytetrafluoroethylene). A reversible phase transition from R-GdOF to O-GdOF was observed around 600 °C upon heating and cooling, which was further confirmed by the in situ X-ray diffraction results. Trivalent rare-earth ion (RE³⁺) doping makes O- and R-GdOF potential bifunctional materials for optical imaging and magnetic resonance imaging (MRI) applications. Under near infrared (NIR) irradiation (980 nm), both RE³⁺-doped O- and R-GdOF exhibit intriguing visible upconversion (UC) emissions with a tunable G/R ratio (RE = Yb/Ho), near single-peak red emissions around 660 nm (RE = Yb/Er) and NIR emissions around 800 nm (RE = Yb/Tm). RE³⁺-doped O- and R-GdOF also show discrepant UC profiles in terms of emission intensity, peak shape and G/R ratios attributed to the subtle symmetry differences of the RE³⁺ doping sites. Besides, the ability of RE³⁺-doped O- and R-GdOF to serve as T₂ weighted MRI contrast agents was evaluated. The results show that R-GdOF performs better than O-GdOF in T₂ weighted MRI with higher r₂ relaxivity values and r₂/r₁ ratio. The exploration of more RE oxyfluorides with multimorphism and the comparative studies of their optical performances will provide deep insight into their structure–property relationship and accelerate the rational design of better multifunctional materials.