Detailed magnetic and structural analysis mapping a robust magnetic C4 dome in Sr1-xNaxFe2As2

The recently discovered C4 tetragonal magnetic phase in hole-doped members of the iron-based superconductors provides insights into the origin of unconventional superconductivity. Previously observed in Ba1-xNaxFe2As2 (with A = K, Na), the C4 magnetic phase exists within the well-studied C2 spin-density-wave dome, arising just before the complete suppression of antiferromagnetic order but after the onset of superconductivity. Here, we present detailed x-ray and neutron diffraction studies of Sr1-xNaxFe2As2 (0.10≤x≤0.60) to determine their structural evolution and the extent of the C4 phase. Spanning Δx∼0.14 in composition, the C4 phase is found to extend over a larger range of compositions, and to exhibit a significantly higher transition temperature, Tr∼65 K, than in either of the other systems in which it has been observed. The onset of this phase is seen near a composition (x∼0.30) where the bonding angles of the Fe2As2 layers approach the perfect 109.46 tetrahedral angle. We discuss the possible role of this return to a higher symmetry environment for the magnetic iron site in triggering the magnetic reorientation and the coupled reentrance to the tetragonal structure. Finally, we present a phase diagram, complete with the C4 phase, and use its observation in a third hole-doped 122 system to suggest the universality of this phase.