K3(FeO2)4 crystallizes in the tetragonal P-4m2 space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 4-coordinate geometry to four equivalent O(2) atoms. In the second K site, K(2) is bonded in a 2-coordinate geometry to two equivalent O(2) atoms. There are three inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to four equivalent O(1) atoms to form distorted corner-sharing FeO4 tetrahedra. In the second Fe site, Fe(2) is bonded to four equivalent O(2) atoms to form corner-sharing FeO4 tetrahedra. In the third Fe site, Fe(3) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form corner-sharing FeO4 tetrahedra. There are two inequivalent O sites. In the first O site, O(1) is bonded in a linear geometry to one Fe(1) and one Fe(3) atom. In the second O site, O(2) is bonded in a distorted linear geometry to one K(1), one K(2), one Fe(2), and one Fe(3) atom.
K3(FeO2)4 crystallizes in the tetragonal P-4m2 space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 4-coordinate geometry to four equivalent O(2) atoms. All K(1)-O(2) bond lengths are 3.25 Å. In the second K site, K(2) is bonded in a 2-coordinate geometry to two equivalent O(2) atoms. Both K(2)-O(2) bond lengths are 3.19 Å. There are three inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to four equivalent O(1) atoms to form distorted corner-sharing FeO4 tetrahedra. All Fe(1)-O(1) bond lengths are 1.83 Å. In the second Fe site, Fe(2) is bonded to four equivalent O(2) atoms to form corner-sharing FeO4 tetrahedra. All Fe(2)-O(2) bond lengths are 1.87 Å. In the third Fe site, Fe(3) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form corner-sharing FeO4 tetrahedra. Both Fe(3)-O(1) bond lengths are 1.86 Å. Both Fe(3)-O(2) bond lengths are 1.86 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded in a linear geometry to one Fe(1) and one Fe(3) atom. In the second O site, O(2) is bonded in a distorted linear geometry to one K(1), one K(2), one Fe(2), and one Fe(3) atom.