The recent discovery of superconductivity in (Ba,K)Fe2As2, which crystallizes in the ThCr2Si2 (122) structure as compared with the LnFeAsO (Ln is lanthanide) systems that possess the ZrCuSiAs (1111) structure, demonstrates the exciting potential of the FeAs-based materials for high-T{C} superconductivity. Here we report neutron diffraction studies that show a tetragonal-to-orthorhombic distortion associated with the onset of q=(101) antiferromagnetic order in BaFe2As2, with a saturation moment 0.87(3)micro {B} per Fe that is orientated along the longer a axis of the ab planes. The simultaneous first-order structural and magnetic transition is in contrast with the separated transitions previously reported in the 1111-type materials. The orientational relation between magnetic alignment and lattice distortion supports a multiorbital nature for the magnetic order.

The relation between the spin-density wave (SDW) and superconducting order is a central topic in the current research on the FeAs-based high-TC superconductors. Conflicting results exist in the LaFeAs(O, F)-class of materials, for which whether the SDW and superconductivity are mutually exclusive or they can coexist has not been settled. Here we show that for the (Ba, K)Fe2As2 system, the SDW and superconductivity can coexist in an extended range of compositions. The availability of single crystalline samples and high value of the energy gaps would make the materials a model system to investigate the high-TC ferropnictide superconductivity.

FeSe-derived superconductors show some unique behaviors relative to iron-pnictide superconductors, which are very helpful to understand the mechanism of superconductivity in high-T c iron-based superconductors. The low-energy electronic structure of the heavily electron-doped A x Fe 2 Se 2 (A=K, Rb, Cs) demonstrates that interband scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors 1,2 . The superconducting transition temperature (T c ) in the one-unit-cell FeSe on SrTiO 3 substrate can reach as high as ~65 K 3-6 , largely transcending the bulk T c of all known iron-based superconductors. However, in the case of A x Fe 2 Se 2 , the inter-grown antiferromagnetic insulating phase makes it difficult to study the underlying physics. Superconductors of alkali metal ions and NH 3 molecules or organic-molecules intercalated FeSe 7-9 and single layer or thin film FeSe on SrTiO 3 substrate 3-6 are extremely air-sensitive, which prevents the further investigation of their physical properties. Therefore, it is urgent to find a stable and accessible FeSe-derived superconductor for physical property measurements so as to study the underlying mechanism of superconductivity. Here, we report the air-stable superconductor (Li 0.8 Fe 0.2 )OHFeSe with high temperature superconductivity at ~40 K synthesized by a novel hydrothermal method. The crystal structure is unambiguously determined by the combination of X-ray and neutron powder diffraction and nuclear magnetic resonance. It is also found that an antiferromagnetic order coexists with superconductivity in such new FeSe-derived superconductor. This novel synthetic route opens a new avenue for exploring other superconductors in the related systems. The combination of different structure characterization techniques helps to complementarily determine and understand the details of the complicated structures