Since its discovery in 2014, the magnetic tetragonal C4 phase has been identified in a growing number of hole-doped 122 Fe-based superconducting compounds. Exhibiting a unique double-Q magnetic structure and a strong competition with both superconducting and magnetic order parameters, the C4 phase and the conditions of its formation are of significant interest to understanding the fundamental mechanisms in these materials. Particularly, separating the importance of direct changes to the relative size of hole and electron pockets at the Fermi surface (achieved via charge doping) from the role of structural changes due to differences of ionic radii of dopants is useful to determine the underlying parameter which causes the C4 instability. Here, we report the discovery of the C4 phase in a fourth member of the hole-doped 122 materials Ca1-xNaxFe2As2(0.20≤x≤0.50) as determined from neutron and x-ray powder diffraction studies. The maximum of the C4 dome is observed at x=0.44 with a reentrant temperature Tr=52 K and an extent of Δx∼0.07 in composition. It is observed that for a range of compositions within the C4 dome (0.40≤x≤0.42), there is a second reentrance (Tr2<Tr) where the antiferromagnetic C2 phase is recovered - a feature previously only seen in Ba1-xKxFe2As2. A phase diagram is presented for Ca1-xNaxFe2As2 and compared to the other Na-doped 122's - A1-xNaxFe2As2 with A = Ba, Sr, and Ca. The structural parameters for these three systems are compared and the importance of the "chemical pressure" due to changing the A-site ion (A = Ba, Sr, Ca) is discussed.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics