The structural and magnetic properties of Cs$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_{2}$ as determined by x ray and neutron scattering of powder and single crystal samples

The A$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_{2}$ family of iron selenides (A $=$ K, Rb and Cs) has proven an intricate system for the study of unconventional superconductivity, exhibiting high temperature superconductivity ($\sim$ 30 K) and a complex structural phase transition into a biphasic state coupled with a high temperature magnetic transition ($\sim$ 500 K). While isostructural to the 122 arsenides, significant structural differences are identified. In the selenides, iron vacancies in the tetrahedral FeSe layers become ordered below a high temperature structural transition defining a main phase $\surd 5 \times \surd 5$ superstructure. Coexistent with the main phase, a secondary phase of a previously contested structure is observed and it is in this biphasic state that superconductivity arises at $\sim$ 30 K. Both powder and single crystal samples show similar phase separation and coexistence. In this talk, I will discuss structural results and lattice parameter evolution obtained from neutron powder diffraction as well as single crystal x-ray diffraction with an emphasis on a novel magnetic structural model, the identification of the secondary phase, and the nature of coincidence of the magnetic, structural and secondary phase transitions.