Spin-density-wave anomaly at 140 K in the ternary iron arsenide BaFe2As2

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, which is 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) . F57e Mössbauer 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 parent compound for oxygen-free iron arsenide superconductors.