SrFeAsF as a parent compound for iron pnictide superconductors

We have successfully synthesized the fluoroarsenide SrFeAsF, which is another parent phase with the ZrCuSiAs structure. The temperature dependence of resistivity and dc magnetization both reveal an anomaly at about T-an = 173 K, which may correspond to the structural and/or spin-density-wave (SDW) transition. Strong Hall effect and moderate magnetoresistance were observed below Tan. Interestingly, the Hall coefficient RH is positive below Tan, which is opposite to the cases of the two parent phases of FeAs-based systems known so far, i.e., LnFeAsO (Ln=rare-earth elements) and (Ba, Sr)Fe2As2, where the Hall coefficient RH is negative. This strongly suggests that the gapping of the Fermi surface induced by the SDW order leaves one of the hole pockets fully or partially ungapped in SrFeAsF. Our data show that it is possible for the parent phases of the arsenide superconductors to display dominant carriers that are either electronlike or holelike.