Superconductivity and crystal chemistry of BaFe2As2 and related 122-materials
Prof. Dr. Dirk Johrendt (Ludwig-Maximilians-Universität München)

The ternary arsenide BaFe2As2 is the parent compound of the iron arsenide superconductors with the tetragonal ThCr2Si2-type structure (122-type). We have discovered superconductivity up to 38 K in hole-doped (Ba1-xKx)Fe2As2 with x = 0.4, quickly followed by reports on the isotypic compounds (Sr1-xKx)Fe2As2 (Tc = 37 K), (Ca1-xNax)Fe2As2 (Tc = 20 K) and (Eu0.5K0.5)Fe2As2 (Tc = 32 K).<p /> Undoped BaFe2As2 shows a spin-density-wave (SDW) anomaly at 140 K similar to LaFeAsO, the parent compound of the 1111-superconductors. The SDW instability is connected with structural and magnetic phase transitions, leading to an antiferromagnetic ground state and to orthorhombic lattice symmetry. In contrast to electron-doped LaFeAsO1-xFx, our phase diagram of hole-doped (Ba1-xKx)Fe2As2 (x = 0-1) shows super¬conductivity that co-exists with the orthorhombic structure and with magnetic ordering according to 57Fe Mössbauer spectra.<p /> Structural features of the ThCr2Si2-Compounds will be presented and discussed with respect to the recently reported so-called collapsed phase of CaFe2As2 . A self doping mechanism is proposed as a possible reason for the occurrence of superconductivity in undoped 122-compounds under pressure.