Credit: Alan Stonebraker

Figure 2: Vacancy ordering and antiferromagnetism in $K0.8Fe1.6Se2$ (the structure shown corresponds to a chemical formula $K2Fe4Se5$). Large circles indicate $Fe$ vacancies, small open ones $Fe$ spin-up sites, filled ones spin-down sites. The figure is not to scale; the bond shortening inside a plaquette is exaggerated.