K3(FeO2)2 crystallizes in the orthorhombic Fddd space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 6-coordinate geometry to two equivalent O(1) and four equivalent O(2) atoms. In the second K site, K(2) is bonded in a 4-coordinate geometry to four equivalent O(1) atoms. Fe(1) 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 to two equivalent K(1), two equivalent K(2), and two equivalent Fe(1) atoms to form a mixture of corner and edge-sharing OK4Fe2 octahedra. The corner-sharing octahedral tilt angles range from 12-65°. In the second O site, O(2) is bonded in a 6-coordinate geometry to four equivalent K(1) and two equivalent Fe(1) atoms.
K3(FeO2)2 crystallizes in the orthorhombic Fddd space group. There are two inequivalent K sites. In the first K site, K(1) is bonded in a 6-coordinate geometry to two equivalent O(1) and four equivalent O(2) atoms. Both K(1)-O(1) bond lengths are 2.71 Å. There are two shorter (2.81 Å) and two longer (2.97 Å) K(1)-O(2) bond lengths. In the second K site, K(2) is bonded in a 4-coordinate geometry to four equivalent O(1) atoms. All K(2)-O(1) bond lengths are 2.70 Å. Fe(1) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form corner-sharing FeO4 tetrahedra. Both Fe(1)-O(1) bond lengths are 2.00 Å. Both Fe(1)-O(2) bond lengths are 1.98 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded to two equivalent K(1), two equivalent K(2), and two equivalent Fe(1) atoms to form a mixture of corner and edge-sharing OK4Fe2 octahedra. The corner-sharing octahedral tilt angles range from 12-65°. In the second O site, O(2) is bonded in a 6-coordinate geometry to four equivalent K(1) and two equivalent Fe(1) atoms.