Hidden Charge Order in an Iron Oxide Square-Lattice Compound

Since the discovery of charge disproportionation in ${\mathrm{FeO}}_{2}$ square-lattice compound ${\mathrm{Sr}}_{3}{\mathrm{Fe}}_{2}{\mathrm{O}}_{7}$ by M\"ossbauer spectroscopy more than fifty years ago, spatial ordering pattern disproportionated charges has remained ``hidden'' to conventional diffraction probes, despite numerous x-ray and neutron scattering studies. We have used Larmor Fe $K$-edge resonant demonstrate checkerboard order planes that vanishes at a sharp second-order phase transition upon heating above 332 K. Stacking disorder due frustrated interlayer interactions broadens corresponding superstructure reflections greatly reduces their amplitude, thus explaining difficulty detecting them probes. discuss implications these findings for research on ``hidden order'' other materials.