  In addition to higher Tc compared with the ubiquitous cuprates for a material
composed of a single electronically active layer, the newly discovered LnFeAsO
superconductors offer additional compositional variation. In a similar fashion
to the CuO2 layers in cuprates, the FeAs layers now dominate the electronic
states that produce superconductivity. Cuprate superconductors distinguish
themselves structurally by adopting different stacking of the Cu-O and
electronically inactive "spacer" layers. Using the same structural philosophy,
materials with the formula (A,K)Fe2As2,A=Ba or Sr have been reported and
possess a Tc~38 K. Here, we report the neutron diffraction studies of BaFe2As2
that shows, in contrast to previous studies on the LnFeAsO materials, an
antiferromagnetic transition which concurs with first-order structural
transition. Although the magnetic and structural transitions occur differently
in the AFe2As2 and LnFeAsO-type materials, this work clearly demonstrates that
the complete evolution to a low symmetry structure is a pre-requirement for the
magnetic order.
