Indifference of Superconductivity and Magnetism to Size-Mismatched Cations in the Layered Iron Arsenides Ba1-xNaxFe2As2

The evolution of the structure, magnetic ordering, and superconductivity in the series Ba1-xNaxFe2As2 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 Bai,Na,Fe,As, members are similar to those of the betterinvestigated Ba1-xNaxFe2As2 analogues. This is evidently a consequence of the quantitatively similar evolution of the structure of the FeAs layers in the two series. In Ba1-xNaxFe2As2 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 Ba1-xNaxFe2As2 series, as judged by the onset of diamagnetism, is 34K in Ba0.6Na0.4Fe2As2. 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.