  The ternary iron arsenide BaFe2As2 with the tetragonal ThCr2Si2-type
structure exhibits a spin density wave (SDW) anomaly at 140 K, very similar to
LaFeAsO, the parent compound of the iron arsenide superconductors. BaFe2As2 is
a poor Pauli-paramagnetic metal and undergoes a structural and magnetic phase
transition at 140 K, accompanied by strong anomalies in the specific heat,
electrical resistance and magnetic susceptibility. In the course of this
transition, the space group symmetry changes from tetragonal (I4/mmm) to
orthorhombic (Fmmm). 57Fe Moessbauer spectroscopy experiments show a single
signal at room temperature and full hyperfine field splitting below the phase
transition temperature (5.2 T at 77 K). Our results suggest that BaFe2As2 can
serve as a new parent compound for oxygen-free iron arsenide superconductors.
