Crystal structure and phase transitions across the metal-superconductor boundary in theSmFeAsO1−xFx(0≤x≤0.20)family

The fluorine-doped rare-earth iron oxyarsenides ${\text{REFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ ($\text{RE}=\text{rare}$ 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 ${\text{SmFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ $(0\ensuremath{\le}x\ensuremath{\le}0.20)$ in which superconductivity emerges near $x\ensuremath{\sim}0.07$ and ${T}_{c}$ increases monotonically with doping up to $x\ensuremath{\sim}0.20$. 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, $x\ensuremath{\ge}0.15$, 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.