The recently discovered C4 tetragonal magnetic phase in hole-doped members of the iron-based superconductors provides new insights into the origin of unconventional superconductivity. Previously observed in Ba1−xAxFe2As2 (with A= K, Na), the C4 magnetic phase exists within the well studied C2 spin-density wave (SDW) dome, arising just before the complete suppression of antiferromagnetic (AFM) 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∼65K, 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 re-entrance to the tetragonal structure. Finally, we present a new 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.
@article{arxiv.1601.05693,
title = {Detailed magnetic and structural analysis mapping a robust magnetic C4 dome in Sr1-xNaxFe2As2},
author = {K. M. Taddei and J. M. Allred and D. E. Bugaris and S. H. Lapidus and M. J. Krogstad and R. Stadel and H. Claus and D. Y. Chung and M. G. Kanatzidis and S. Rosenkranz and R. Osborn and O. Chmaissem},
journal= {arXiv preprint arXiv:1601.05693},
year = {2016}
}