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Superconductivity, magnetism and crystal chemistry of Ba1-xKxFe2As2

BaFe2As2 is the parent compound of the '122' iron arsenide superconductors and crystallizes with the tetragonal ThCr2Si2 type structure, space group I4/mmm. A spin density wave transition at 140 K is accompanied by a symmetry reduction to space group Fmmm and simultaneously by antiferromagnetic ordering. Hole-doping induces superconductivity in Ba1-xKxFe2As2 with a maximum Tc of 38 K at x = 0.4. The upper critical fields approach 75 T with rather small anisotropy of Hc2. At low potassium concentrations (x <= 0.2), superconductivity apparently co-exists with the orthorhombic distorted and magnetically ordered phase. At doping levels x >= 0.3, the structural distortion and antiferromagnetic ordering is completely suppressed and the Tc is maximized. No magnetically ordered domains could be detected in optimally doped Ba1-xKxFe2As2 (x >= 0.3) by 57Fe-Moessbauer spectroscopy in contrast muSR results obtained with single crystals. The magnetic hyperfine interactions investigated by 57Fe Moessbauer spectroscopy are discussed and compared to the ZrCuSiAs-type materials.