Observation of the magnetic C4 phase in Ca1−xNaxFe2As2 and its universality in the hole-doped 122 superconductors

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Since its discovery in 2014, the magnetic tetragonal ${C}_{4}$ 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 ${C}_{4}$ 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 ${C}_{4}$ instability. Here, we report the discovery of the ${C}_{4}$ phase in a fourth member of the hole-doped 122 materials ${\mathrm{Ca}}_{\text{1-x}}{\mathrm{Na}}_{\text{x}}{\mathrm{Fe}}_{\text{2}}{\mathrm{As}}_{\text{2}}\phantom{\rule{4pt}{0ex}}(0.20\ensuremath{\le}x\ensuremath{\le}0.50)$ as determined from neutron and x-ray powder diffraction studies. The maximum of the ${C}_{4}$ dome is observed at $x=0.44$ with a reentrant temperature ${T}_{r}=52$ K and an extent of $\mathrm{\ensuremath{\Delta}}x\ensuremath{\sim}0.07$ in composition. It is observed that for a range of compositions within the ${C}_{4}$ dome $(0.40\ensuremath{\le}x\ensuremath{\le}0.42$), there is a second reentrance $({T}_{{r}_{2}}\phantom{\rule{4pt}{0ex}}l\phantom{\rule{4pt}{0ex}}{T}_{r})$ where the antiferromagnetic ${C}_{2}$ phase is recovered---a feature previously only seen in ${\mathrm{Ba}}_{\text{1-x}}{\mathrm{K}}_{\text{x}}{\mathrm{Fe}}_{\text{2}}{\mathrm{As}}_{\text{2}}$. A phase diagram is presented for ${\mathrm{Ca}}_{\text{1-x}}{\mathrm{Na}}_{\text{x}}{\mathrm{Fe}}_{\text{2}}{\mathrm{As}}_{\text{2}}$ and compared to the other $\mathrm{Na}$-doped 122's---${A}_{\text{1-x}}{\mathrm{Na}}_{\text{x}}{\mathrm{Fe}}_{\text{2}}{\mathrm{As}}_{\text{2}}$ 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.