Reversible Exsolution of Nanometric Fe<sub>2</sub>O<sub>3</sub> Particles in BaFe<sub>2–<i>x</i></sub>(PO<sub>4</sub>)<sub>2</sub> (0 ≤ <i>x</i> ≤ 2/3): The Logic of Vacancy Ordering in Novel Metal-Depleted Two-Dimensional Lattices AlcoverIgnacio Blazquez DavidRénald Daviero-MinaudSylvie FilimonovDmitry HuvéMarielle RousselPascal KabbourHouria MentréOlivier 2015 We show here that the exsolution of Fe<sup>2+</sup> ions out of two-dimensional (2D) honeycomb layers of BaFe<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub> into iron-deficient BaFe<sub>2–<i>x</i></sub>­(PO<sub>4</sub>)<sub>2</sub> phases and nanometric α-Fe<sub>2</sub>O<sub>3</sub> (typically 50 nm diameter at the grain surface) is efficient and reversible until <i>x</i> = 2/3 in mild oxidizing/reducing conditions. It corresponds to the renewable conversion of 12 wt % of the initial mass into iron oxide. After analyzing single crystal X-ray diffraction data of intermediate members <i>x</i> = 2/7, <i>x</i> = 1/3, <i>x</i> = 1/2 and the ultimate Fe-depleted <i>x</i> = 2/3 term, we observed a systematic full ordering between Fe ions and vacancies (V<sub>Fe</sub>) that denote unprecedented easy in-plane metal diffusion driven by the Fe<sup>2+</sup>/Fe<sup>3+</sup> redox. Besides the discovery of a diversity of original depleted triangular <sub>∞</sub>{Fe<sup>2/3+</sup><sub>2–<i>x</i></sub>O<sub>6</sub>} topologies, we propose a unified model correlating the <i>x</i> Fe-removal and the experimental Fe/V<sub>Fe</sub> ordering into periodic one-dimensional motifs paving the layers, gaining insights into predictive crystal chemistry of complex low dimensional oxides. Increasing the <i>x</i> values led to a progressive change of the materials from 2D ferromagnets (Fe<sup>2+</sup>) to 2D ferrimagnets (Fe<sup>2/3+</sup>) to antiferromagnets for <i>x</i> = 2/3 (Fe<sup>3+</sup>).