Single crystals of the title compound, potassium hexa-phosphito-penta-ferrate(II,III) hemihydrate, K0.75[Fe(II) 3.75Fe(III) 1.25(HPO3)6]·0.5H2O, were grown under mild hydro-thermal conditions. The crystal structure is isotypic with Li1.43[Fe(II) 4.43Fe(III) 0.57(HPO3)6]·1.5H2O and (NH4)2[Fe(II) 5(HPO3)6] and exhibits a [Fe(II) 3.75Fe(III) 1.25(HPO3)6](0.75-) open framework with disordered K(+) (occupancy 3/4) as counter-cations. The anionic framework is based on (001) sheets of two [FeO6] octa-hedra (one with point group symmetry 3.. and one with point group symmetry .2.) linked along [001] through [HPO3](2-) oxoanions. Each sheet is constructed from 12-membered rings of edge-sharing [FeO6] octa-hedra, giving rise to channels with a radius of ca 3.1 Å where the K(+) cations and likewise disordered water mol-ecules (occupancy 1/4) are located. O⋯O contacts between the water mol-ecule and framework O atoms of 2.864 (5) Å indicate hydrogen-bonding inter-actions of medium strength. The infrared spectrum of the compound shows vibrational bands typical for phosphite and water groups. The Mössbauer spectrum is in accordance with the presence of Fe(II) and Fe(III) ions.
Keywords: crystal structure; hydrothermal synthesis; isotypism; mixed-valent FeII/FeIII compound; open-framework structure.