Spin-density-wave anomaly at 140 K in the ternary iron arsenide ${\text{BaFe}}_2{\text{As}}_2$

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.

Figure 1

Crystal structures of LaFeAsO (left, ZrCuSiAs type) and ${\text{BaFe}}_2{\text{As}}_2$ (right, ${\text{ThCr}}_2{\text{Si}}_2$ type).

Figure 2

(Color online) X-ray powder pattern $(+)$ and Rietveld fit $(-)$ of ${\text{BaFe}}_2{\text{As}}_2$ at 297 K (space group $I4/mmm$).

Figure 3

Specific heat of ${\text{BaFe}}_2{\text{As}}_2$ vs temperature.

Figure 4

(Color online) X-ray powder pattern $(+)$ and Rietveld fit $(-)$ of ${\text{BaFe}}_2{\text{As}}_2$ at 20 K (space group $Fmmm$).

Figure 5

Splitting of the 110 and 112 reflections and variation of the lattice parameters with temperature. Values for the tetragonal phase above 140 K are multiplied by $\sqrt{2}$ for comparability.

Figure 6

dc electrical resistance of ${\text{BaFe}}_2{\text{As}}_2$ $\left(I=100\mu \text{A}\right)$. Inset. Magnetic susceptibility measured at 0.5 T.

Figure 7

(Color online) ${}^{57}\text{F}\text{e}$ Mössbauer spectra of ${\text{BaFe}}_2{\text{As}}_2$ with transmission integral fits.