Ba_1-xRb_xFe₂As₂ and generic phase behavior of hole-doped 122-type superconductors

The solid solution Ba_1-xRb_xFe₂As₂ (0 ≤ x ≤ 1) was synthesized by solid state reactions and characterized by X-ray powder and single crystal diffraction. The compounds crystallize in the ThCr₂Si₂-type structure (space group I4/mmm) and contain layers of edge-sharing FeAs_4/4-tetrahedra without bonds between the layers. Rubidium substitution decreases the temperature of the structural phase transition in BaFe₂As₂ (I4/mmm → Fmmm) until the orthorhombic distortion is completely suppressed at x = 0.33. Superconductivity already emerges at x = 0.15, reaches the highest critical temperature of 37.5 K close to x = 0.4 and exists up to x = 1. The T_c(x) phase diagram of Ba_1-xRb_xFe₂As₂ is remarkably similar to those of Ba_1-xK_xFe₂As₂ and Ba_1-xNa_xFe₂As₂ in spite of opposing volume effects. The highest critical temperature coincides with an almost ideal As-Fe-As bond angle (109.5 ) in the FeAs₄-tetrahedra. The reduced critical temperatures T_c/T_{c, max} of the so far known hole-doped 122-type superconductors exhibit a generic behavior, where the highest transition temperatures occur near 0.19 holes per iron, which is comparable to the hole-doped copper-oxides, where the maximum is near 0.16 holes per copper. Such remarkably similar values in the two classes of high-T_c superconductors may not be fortuitous, and suggest parallels of both systems concerning the unresolved mechanism of unconventional superconductivity. (Copyright copyright 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)