  We use neutron scattering to study the spin and lattice structure on single
crystals of SrFe2As2, the parent compound of the FeAs based superconductor
(Sr,K)Fe2As2. We find that SrFe2As2 exhibits an abrupt structural phase
transitions at 220K, 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 in SrFe2As2 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 elements) families of materials and on BaFe2As2, and therefore suggest
that static antiferromagnetic order is ubiquitous for the parent compound of
these FeAs-based high-transition temperature superconductors.
