Structural, magnetic, and superconducting properties of Ba _1-xNa_xFe₂As₂

We report the results of a systematic investigation of the phase diagram of the iron-based superconductor system, Ba_1-xNa_xFe ₂As₂, from x=0.1 to x=1.0 using high-resolution neutron and x-ray diffraction and magnetization measurements. We find that the coincident structural and magnetic phase transition to an orthorhombic structure with space group Fmmm and a striped antiferromagnet with space group F _Cmm^′m^′ in Ba_1-xNa _xFe₂As₂ is of first order. A complete suppression of the magnetic phase is observed by x = 30%, and bulk superconductivity occurs at a critical concentration near 15%. We compare our findings to the previously reported results of the hole-doped Ba _1-xK_xFe₂As₂ solid solution in order to resolve the differing effects of band filling and A-site cation size on the properties of the magnetic and superconducting ground states. The substantial size difference between Na and K causes various changes in the lattice trends, yet the overarching property phase diagram from the Ba_1-xK _xFe₂As₂ phase diagram carries over to the Ba_1-xNa_xFe₂As₂ solid solution. We note that the composition dependence of the c axis turns over from positive to negative around x = 0.35, unlike the K-substituted materials. We show that this can be understood by invoking steric effects; primarily the Fe ₂As₂ layer shape is dictated mostly by the electronic filling, which secondarily induces an interlayer spacing adjusted to compensate for the given cation volume. This exemplifies the primacy of even subtle features in the Fe₂As₂ layer in controlling both the structure and properties in the uncollapsed 122 phases.