Ba1-xRbxFe2As2 and Generic Phase Behavior of Hole-doped 122-Type Superconductors

The solid solution Ba1–xRbxFe2As2 (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 FeAs4/4‐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 Tc(x) phase diagram of Ba1–xRbxFe2As2 is remarkably similar to those of Ba1–xKxFe2As2 and Ba1–xNaxFe2As2 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 Tc/Tc, 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‐Tc superconductors may not be fortuitous, and suggest parallels of both systems concerning the unresolved mechanism of unconventional superconductivity.