Ammonium iron(III) phosphate(V) fluoride, (NH(4))(0.5)[(NH(4))(0.375)K(0.125)]FePO(4)F, with ammonium partially substituted by potassium.

The title compound, ammonium potassium iron(III) phosphate fluoride, (NH(4))(0.875)K(0.125)FePO(4)F, is built from zigzag chains (∞) (1){[FeO(4)F(2)](7-)}, with Fe(3+) in a distorted octahedral coordination, extending along both the [011] and [01] directions. These chains are made up of alternating trans-[FeO(4)F(2)] and cis-[FeO(4)F(2)] octa-hedra via shared F-atom corners, and are linked by PO(4) tetra-hedra, resulting in an open-framework structure with channels along the [010] and [100] directions. There are two crystallographically independent ammonium sites: one in the [010] channels and the other, partially substituted by K(+) ions, in the [100] channels. The ammonium in the [010] channels is fixed to the framework via eight hydrogen bonds (six N-H⋯O and two N-H⋯F).