We report the results of a systematic investigation of the phase diagram of the iron-based superconductor system, Ba 1−x Na x Fe 2 As 2 , 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 F mmm and a striped antiferromagnet with space group F C mm m in Ba 1−x Na x Fe 2 As 2 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−x K x Fe 2 As 2 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−x K x Fe 2 As 2 phase diagram carries over to the Ba 1−x Na x Fe 2 As 2 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 2 As 2 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 Fe 2 As 2 layer in controlling both the structure and properties in the uncollapsed 122 phases.

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Iron selenide (FeSe x ) crystals with lateral dimensions up to millimeters were grown via a vapor self-transport method. The crystals consist of the dominant α -phase with trace amounts of β -phase as identified by powder x-ray diffraction. With four-probe resistance measurements we obtained a zero-resistance critical temperature of 7.5 K and a superconducting onset transition temperature of up to 11.8 K in zero magnetic field as well as an anisotropy of 1.5 ± 0.1 for the critical field. Magnetization measurements on individual crystals reveal the co-existence of superconductivity and ferromagnetism.

Combined neutron and x-ray diffraction experiments demonstrate the formation of a lowtemperature minority magnetic tetragonal phase in Ba0.76K0.24Fe2As2 in addition to the majority magnetic, orthorhombic phase. The coincident enhancement in the magnetic ( 1 2 1 2 1) peaks shows that this minority phase is of the same type that was observed in Ba1−xNaxFe2As2 (0.24 ≤ x ≤ 0.28), in which the magnetic moments reorient along the c-axis. This is evidence that the tetragonal magnetic phase is a universal feature of the hole-doped iron-based superconductors. The observations suggest that in this regime the energy levels of the C2 and C4 symmetric magnetic phases are very close.