Single crystals of Ca₂FeO₃Cl have been obtained as a by product during single crystal growth experiments of calcium ferrates from a CaCl₂ flux. The reddish-brown optically uni-axial crystals adopt the tetragonal space group P4/nmm with a = 3.8381(4) Å and c = 13.685(2) Å and Z = 2 formula units per cell. The structure has been determined from a single crystal diffraction data set collected at room conditions and refined to final residual R(|F|) = 0.053 for 163 observed independent reflections with I > 2 σ (I). Ca₂FeO₃Cl belongs to the structure family of the Ruddlesden-Popper series with n = 1, which is also referred to as the K₂NiF₄-type. Main building units are layers of perovskite type corner connected FeO₅Cl-octahedra perpendicular to [001]. The two crystallographically independent calcium ions are located between the octahedral layers and are coordinated by nine ligands each: Ca1 (4xO + 5xCl) and Ca2 (9xO). Following prior studies Ca₂FeO₃Cl crystallizes in space group P4. However, the present investigation shows clearly that this assignment is incorrect and that the compound has been described in an unnecessarily low symmetry.

Keywords: ruddlesden-popper phase, symmetry

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