  We report the results of a systematic investigation of the phase diagram of
the iron-based superconductor system, Ba1-xNaxFe2As2, 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 (O) structure with space group Fmmm and a striped
antiferromagnet (AF) with space group F(C)mm'm' in Ba1-xNaxFe2As2 is of first
order. A complete suppression of the magnetic phase is observed by x = 0.30,
and bulk superconductivity occurs at a critical concentration near 0.15. We
compare the new findings to the previously reported results of the hole-doped
Ba1-xKxFe2As2 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 Ba1-xKxFe2As2 phase diagram carries over to the
Ba1-xNaxFe2As2 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 Fe2As2 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.
