Spin and lattice structures of single-crystalline SrFe2As2

We use neutron scattering to study the spin and lattice structure of single-crystal SrFe2As2, the parent compound of the FeAs-based superconductor (Sr,K)Fe2As2. We find that SrFe2As2 exhibits an abrupt structural phase transition at 220 K, where the structure changes from tetragonal with lattice parameters c>a=b to orthorhombic with c>a>b. At almost the same temperature, Fe spins develop a collinear antiferromagnetic structure along the orthorhombic a axis with spin direction parallel to this a axis. These results are consistent with earlier work on the RFeAsO (R=rare earth) families of materials and on BaFe2As2, and therefore suggest that static antiferromagnetic order is ubiquitous for the parent compounds of these FeAs-based high-transition temperature superconductors.