  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.
