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

The fluorine-doped rare-earth iron oxyarsenides REFeAsO1-xFx (RE=rare earth) have recently emerged as a new family of high-temperature superconductors with transition temperatures (T-c) as high as 55 K. Here we use high-resolution synchrotron x-ray diffraction to study the structural properties of SmFeAsO1-xFx (0 <= x <= 0.20) in which superconductivity emerges near x similar to 0.07 and T-c increases monotonically with doping up to x similar to 0.20. We find that orthorhombic symmetry survives through the metal-superconductor boundary well into the superconducting regime and the structural distortion is only suppressed at doping levels, x >= 0.15, when the superconducting phase becomes metrically tetragonal. Remarkably this crystal symmetry crossover coincides with reported drastic anomalies in the resistivity and the Hall coefficient, and a switch of the pressure coefficient of T-c from positive to negative, thereby implying that the low-temperature structure plays a key role in defining the electronic properties of these superconductors.