Crystal structure and phase transitions across the metal-superconductor boundary in the SmFeAsO_1−xF_x (0≤x≤0.20) family

The fluorine-doped rare-earth iron oxyarsenides REFeAsO_1−xF_x (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 SmFeAsO_1−xF_x (0≤x≤0.20) in which superconductivity emerges near x∼0.07 and T_c increases monotonically with doping up to x∼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.