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The fluorine-doped rare-earth iron oxyarsenides, REFeAsO1-xFx (RE =rareearth) have recently emerged as a new family of high-temperaturesuperconductors with transition temperatures (Tc) as high as 55 K (refs 1-4).Early work has provided compelling evidence that the undoped parent materialsexhibit spin-density-wave (SDW) antiferromagnetic order and undergo astructural phase transition from tetragonal to orthorhombic crystal symmetryupon cooling.5 Both the magnetic and structural instabilities are suppressedupon doping with fluoride ions before the appearance of superconductivity.6,7Here we use high-resolution synchrotron X-ray diffraction to study thestructural properties of SmFeAsO1-xFx (0 < x < 0.20) in which superconductivityemerges near x ~ 0.07 and Tc increases monotonically with doping up to x ~0.20.8 We find that orthorhombic symmetry survives through themetal-superconductor boundary well into the superconducting regime 2 and thestructural distortion is only suppressed at doping levels, x > 0.15 when thesuperconducting phase becomes metrically tetragonal. Remarkably this crystalsymmetry crossover coincides with reported drastic anomalies in the resistivityand the Hall coefficient8 and a switch of the pressure coefficient of Tc frompositive to negative,9 thereby implying that the low-temperature structureplays a key role in defining the electronic properties of thesesuperconductors.

Crystal structure and phase transitions across the metal-superconductor boundary in the SmFeAsO1-xFx (0 < x < 0.20) family