Intrinsic crystal phase separation and detailed structural characterization in Cs$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_2$ superconductor via high resolution diffraction

The discovery of high critical temperature superconductivity in complex metal cuprate pnictide and chalcogenide compounds is a major breakthrough in materials synthesis and in developing new concepts, compounds and technologies. The mechanisms of charge carrier density control are important as small changes in composition produce metal-insulator transitions and generate superconductivity at temperatures of up to 37K in chalcogenides. Reported materials are based on a square FeSe layer built from edge-sharing of FeSe$_4$ tetrahedra. Insertion of alkali metal cations between FeSe layers affords superconductivity in this system. We have grown Cs-intercalated FeSe samples that show superconductivity with different Tc between 10K and 28K by changing the iron and cesium concentration in the nominal composition Cs$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_2$ (0.7 \textless\ x \textless\ 1.1, 0 \textless\ y \textless\ 0.7). These are two phase systems and only one phase is SC. The relationship between structural and superconducting properties will be discussed based on high-resolution X-ray diffraction and single-crystal X-ray measurements combined with magnetometry, heat capacity, and transport measurements.