  We report on doping dependencies of structural parameters and superconducting
transition temperatures in the solid solution (Ba1-xKx)Fe2As2. As the main
effect of doping on the crystal structure, we find linear decreasing As-Fe-As
bond angles and Fe-Fe distances, equivalent to an elongation of the FeAs4
tetrahedra along [001]. The structural changes are intimately coupled to the
electronic states at the Fermi level, because the most relevant Fe-3dx2-y2
orbitals are strongly affected by the As-Fe-As bond angle. Superconductivity is
present over the whole doping range in (Ba1-xKx)Fe2As2 with a maximum Tc of 38
K at x ~ 0.4. The superconducting transitions in the orthorhombic compounds
(Ba0.9K0.1)Fe2As2 (Tc ~ 3 K) and (Ba0.8K0.2)Fe2As2 (Tc ~ 25 K) is strong
evidence for the coexistence of superconductivity with the structurally
distorted and potentially magnetically ordered state in the BaFe2As2 family of
iron arsenide superconductors.
