The ternary iron arsenide BaFe2As2 becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity at Tc = 38 K in (Ba1−xKx)Fe2As2 with x ≈ 0.4. The parent compound BaFe2As2 as well as KFe2As2 both crystallize in the tetragonal ThCr2Si2-type structure, which consists of (FeAs) δ− iron arsenide layers separated by barium or potassium ions. BaFe2As2 is a poor metal and exhibits a spin density wave (SDW) anomaly at 140 K. By substituting Ba 2+ for K + ions we have introduced holes in the (FeAs) − layers, which suppress the SDW anomaly and induce superconductivity. This scenario is very similar to the recently discovered arsenide-oxide superconductors. The Tc of 38 K in (Ba0.6K0.4)Fe2As2 is the highest critical temperature in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ThCr2Si2-type structure. Our results suggest, that superconductivity in these systems evolves essentially from the (FeAs) δ− layers and may occur in other related compounds.

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 (F mmm). 57 Fe 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.

Doping improves performance: Iron arsenides (Ba1−xKx)Fe2As2 with the ThCr2Si2‐type structure exhibit superconductivity at 3–38 K depending on the potassium doping level. Superconductivity occurs before the structural distortion of the parent compound BaFe2As2 (x=0) is completely suppressed by doping (see phase diagram; • critical temperature, ○ phase‐transition temperature). Doping decreases the bond angles in the iron arsenide layers, suggesting a strong coupling of structural and electronic degrees of freedom.

The structural and magnetic phase transitions of the ternary iron arsenides SrFe 2 As 2 and EuFe 2 As 2 were studied by temperature-dependent x-ray powder diffraction and 57 Fe Mössbauer spectroscopy. Both compounds crystallize in the tetragonal ThCr 2 Si 2 -type structure at room temperature and exhibit displacive structural transitions at 203 K (SrFe 2 As 2 ) or 190 K (EuFe 2 As 2 ) to orthorhombic lattice symmetry in agreement with the group-subgroup relationship between I4/mmm and F mmm. 57 Fe Mössbauer spectroscopy experiments with SrFe 2 As 2 show full hyperfine field splitting below the phase transition temperature (8.91(1) T at 4.2 K). Order parameters were extracted from detailed measurements of the lattice parameters and fitted to a simple power law. We find a relation between the critical exponents and the transition temperatures for AFe 2 As 2 compounds, which shows that the transition of BaFe 2 As 2 is indeed more continuous than the transition of SrFe 2 As 2 but it remains second order even in the latter case.

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