Synthesis, crystal structure and spin-density-wave anomaly of the iron   arsenide-fluoride SrFeAsF
  The new quaternary iron arsenide-fluoride SrFeAsF with the tetragonal ZrCuSiAs-type structure was synthesized and the crystal structure was determined by X-ray powder diffraction (P4/nmm, a = 399.30(1), c = 895.46(1) pm). SrFeAsF undergoes a structural and magnetic phase transition at 175 K, accompanied by strong anomalies in the specific heat, electrical resistance and magnetic susceptibility. In the course of this transition, the space group symmetry changes from tetragonal (P4/nmm) to orthorhombic (Cmme). 57Fe Moessbauer spectroscopy experiments show a single signal at room temperature at an isomer shift of 0.30(1) mm/s and magnetic hyperfine-field splitting below the phase transition temperature. Our results clearly show that SrFeAsF exhibits a spin density wave (SDW) anomaly at 175 K very similar to LaFeAsO, the parent compound of the iron arsenide-oxide superconductors and thus SrFeAsF may serve as a further parent compound for oxygen-free iron arsenide superconductors. 
M. Tegel, S. Johansson, V. Weiss, I. Schellenberg, W. Hermes, R.   Poettgen and Dirk Johrendt