Structure and superconductivity in the 1144 type compounds of AeAFe$_{\mathrm{4}}$As$_{\mathrm{4}}$ (Ae $=$ Ca, Sr, A $=$ K, Rb, Cs)

The Fe-based superconductors$^{\mathrm{\thinspace }}$discovered in 2008 have attracted significant research interest, because of their rich material variety as well as their high superconducting transition temperatures (T$_{\mathrm{c}})$. A large number of related Fe-based superconductors with various types of crystal structures have been found to date. Here, we report new-structure-type Fe-based superconductors AeAFe$_{\mathrm{4}}$As$_{\mathrm{4}}$ (Ae $=$ Ca, Sr, A $=$ K, Rb, or Cs), which can be regarded as hybrid phases between AeFe$_{\mathrm{2}}$As$_{\mathrm{2}}$ and AFe$_{\mathrm{2}}$As$_{\mathrm{2}}$. Unlike solid solutions such as (Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}})$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ and (Sr$_{\mathrm{1-x}}$Na$_{\mathrm{x}})$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$, the Ae and A do not occupy crystallographically equivalent sites, owing to large differences between their ionic radii. Rather, the Ae and A layers are inserted alternately between the Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ layers in the c-axis direction in AeAFe$_{\mathrm{4}}$As$_{\mathrm{4}}$ (AeA1144). The ordering of the Ae and A layers causes a change in space group from I4/mmm to P4/mmm, which is clearly apparent in powder X-ray diffraction patterns. AeA1144 is the first known structure among not only Fe-based superconductors, but also other materials. The AeA1144 is formed as a stoichiometric compound. Therefore, each AeA1144 has its own T$_{\mathrm{c}}$ of approximately 31$-$36 K.