The title compound, hexapotassium octairon(II,III) dodecaphosphonate, exhibiting a two-dimensional structure, is a new mixed alkali/3d metal phosphite. It crystallizes in the space group R3m, with two crystallographically independent Fe atoms occupying sites of 3m (Fe1) and 3m (Fe2) symmetry. The Fe2 site is fully occupied, whereas the Fe1 site presents an occupancy factor of 0.757 (3). The three independent O atoms, one of which is disordered, are situated on a mirror and all other atoms are located on special positions with 3m symmetry. Layers of formula [Fe3(HPO3)4](2-) are observed in the structure, formed by linear Fe3O12 trimer units, which contain face-sharing FeO6 octahedra interconnected by (HPO3)(2-) phosphite oxoanions. The partial occupancy of the Fe1 site might be described by the formation of two [Fe(HPO3)2](-) layers derived from the [Fe3(HPO3)4](2-) layer when the Fe1 atom is absent. Fe(2+) is localized at the Fe1 and Fe2 sites of the [Fe3(HPO3)4](2-) sheets, whereas Fe(3+) is found at the Fe2 sites of the [Fe(HPO3)2](-) sheets, according to bond-valence calculations. The K(+) cations are located in the interlayer spaces, between the [Fe3(HPO3)4](2-) layers, and between the [Fe3(HPO3)4](2-) and [Fe(HPO3)2](-) layers.
Keywords: crystal structure; hydrothermal synthesis; mixed-valence iron phosphite; open-framework structure; trigonal space group.