Spin-density-wave anomaly at 140 K in the ternary iron arsenideBaFe2As2

The ternary iron arsenide ${\text{BaFe}}_2{\text{As}}_2$, with the tetragonal ${\text{ThCr}}_2{\text{Si}}_2$-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. ${\text{BaFe}}_2{\text{As}}_2$ 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 $\left(I4/mmm\right)$ to orthorhombic $\left(Fmmm\right)$. ${}^{57}\text{F}\text{e}$ 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 ${\text{BaFe}}_2{\text{As}}_2$ can serve as a parent compound for oxygen-free iron arsenide superconductors.