Indifference of Superconductivity and Magnetism to Size-Mismatched Cations in the Layered Iron Arsenides Ba_1−xNa_xFe₂As₂

The evolution of the structure, magnetic ordering, and superconductivity in the series Ba 1− x Na x Fe 2 As 2 is reported up to the limiting Na-rich composition with x = 0.6; the more Na-rich compositions are unstable at high temperatures with respect to competing phases. The magnetic and superconducting behaviors of the Ba 1− x Na x Fe 2 As 2 members are similar to those of the better-investigated Ba 1− x K x Fe 2 As 2 analogues. This is evidently a consequence of the quantitatively similar evolution of the structure of the FeAs layers in the two series. In Ba 1− x Na x Fe 2 As 2 antiferromagnetic order and an associated structural distortion are evident for x ≤ 0.35 and superconductivity is evident when x exceeds 0.2. For 0.4 ≤ x ≤ 0.6 bulk superconductivity is evident, and the long-range antiferromagnetically ordered state is completely suppressed. The maximum T c in the Ba 1− x Na x Fe 2 As 2 series, as judged by the onset of diamagnetism, is 34 K in Ba 0.6 Na 0.4 Fe 2 As 2 . Despite the large mis-match in sizes between the two electropositive cations which separate the FeAs layers, there is no evidence for ordering of these cations on the length scale probed by electron diffraction.