BaFe2As2

BaFe2As2 is an iron-based compound that serves as the parent member of the 122 family of iron pnictide superconductors. It crystallizes in the tetragonal ThCr2Si2-type structure (space group I4/mmm) consisting of alternating layers of edge-sharing FeAs tetrahedra forming FeAs planes separated by layers of Ba ions.

At ambient pressure, BaFe2As2 undergoes a structural transition from tetragonal to orthorhombic around 140 K, which is accompanied by the onset of a spin-density-wave antiferromagnetic order on the Fe sublattice. This magnetostructural transition partially gaps the Fermi surface and competes with superconductivity.

Superconductivity is not intrinsic at ambient conditions, but can be induced by chemical substitution or pressure. Hole doping by potassium substitution on the Ba site (Ba1-xKxFe2As2) yields superconductivity with a maximum Tc around 38 K. Electron doping by substituting Co for Fe (Ba(Fe1-xCox)2As2) also yields superconductivity with Tc up to the low 20s K. Application of external pressure can similarly induce superconductivity in some compositions, often suppressing the magnetic and structural transitions and enabling a superconducting state.

BaFe2As2 and its doped variants are central to studies of high-temperature superconductivity in iron pnictides, illustrating the interplay between magnetism, structure, and superconductivity. High-quality single crystals are produced for transport, magnetic, and spectroscopic investigations, and the 122 family has become a major platform for exploring pairing mechanisms in unconventional superconductors.