Figure 2: Vacancy ordering and antiferromagnetism in ${\text{K}}_{0.8}{\text{Fe}}_{1.6}{\text{Se}}_2$ (the structure shown corresponds to a chemical formula ${\text{K}}_2{\text{Fe}}_4{\text{Se}}_5$). Large circles indicate $\text{Fe}$ vacancies, small open ones $\text{Fe}$ spin-up sites, filled ones spin-down sites. The figure is not to scale; the bond shortening inside a plaquette is exaggerated.