Cs vacancy ordering and properties of phase separated Cs$_{x}$Fe$_{2-y}$Se$_{2}$*

Iron-based selenides are among the most complex and least understood superconductors. At high temperature, a \textsc{\char13}122\textsc{\char13}-type structure with random iron vacancies undergoes a complex iron vacancy ordering scheme below $\sim$500K causing the material to phase separate into A$_{2}$Fe$_{4}$Se$_{5}$, known as the 245 phase, and a minority A-site deficient and fully iron stoichiometric A$_{x}$Fe$_{2}$Se$_{2}$ phase (122). At slightly lower temperatures, the material undergoes another transition with the Fe spins of the main \textsc{\char13}245\textsc{\char13} phase ordering into an exotic checkerboard-type magnetic structure with a large magnetic moment. The minority 122 phase is reported to either remain nonmagnetic or to become magnetic below $\sim$200K. At temperatures below $\sim$30K, the magnetic material becomes superconducting and the two states appear to coexist. I will present and discuss our recent synthesis and characterization of high quality Cs$_{x}$Fe$_{2-y}$Se$_{2}$ single crystals and bulk samples with various Tc\textsc{\char13}s that form a relatively large superconducting dome. I will discuss our findings of a previously unseen three dimensional cesium vacancy ordering in the low temperature 122 phase in addition to hosting superconductivity.