Ba_1–<i>x</i>Rb<i>_x</i>Fe₂As₂ and Generic Phase Behavior of Hole‐doped 122‐Type Superconductors

Abstract

Abstract The solid solution Ba 1– x Rb x Fe 2 As 2 (0 ≤ x ≤ 1) was synthesized by solid state reactions and characterized by X‐ray powder and single crystal diffraction. The compounds crystallize in the ThCr 2 Si 2 ‐type structure (space group I 4/ 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 BaFe 2 As 2 ( I 4/ 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– x Rb x Fe 2 As 2 is remarkably similar to those of Ba 1– x K x Fe 2 As 2 and Ba 1– x Na x Fe 2 As 2 in spite of opposing volume effects. The highest critical temperature coincides with an almost ideal As–Fe–As bond angle (109.5°) in the FeAs 4 ‐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.