Two-dimensional Cs-vacancy superstructure in iron-based superconductorCs0.8Fe1.6Se2

Abstract

Single crystal neutron diffraction is combined with synchrotron x-ray scattering to identify the different superlattice phases present in ${\text{Cs}}_{0.8}{\text{Fe}}_{1.6}{\text{Se}}_{2}$. A combination of single crystal refinements and first principles modeling are used to provide structural solutions for the $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ and $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ superlattice phases. The $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ superlattice structure is predominantly composed of ordered Fe vacancies and Fe distortions, whereas the $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ superlattice is composed of ordered Cs vacancies. The Cs vacancies only order within the plane, causing Bragg rods in reciprocal space. By mapping x-ray diffraction measurements with narrow spatial resolution over the surface of the sample, the structural domain pattern was determined, consistent with the notion of a majority antiferromagnetic $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ phase and a superconducting $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ phase.