FeO2 crystallizes in the trigonal R3m space group. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(2) and three equivalent O(1) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 57-59°. In the second Fe site, Fe(2) is bonded to one O(2), one O(3), two equivalent O(1), and two equivalent O(4) atoms to form FeO6 octahedra that share corners with three equivalent Fe(1)O4 tetrahedra and edges with four equivalent Fe(2)O6 octahedra. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Fe(1) and two equivalent Fe(2) atoms. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Fe(1) and three equivalent Fe(2) atoms. In the third O site, O(3) is bonded in a distorted T-shaped geometry to three equivalent Fe(2) atoms. In the fourth O site, O(4) is bonded in a water-like geometry to two equivalent Fe(2) atoms.
FeO2 crystallizes in the trigonal R3m space group. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(2) and three equivalent O(1) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 57-59°. The Fe(1)-O(2) bond length is 1.93 Å. All Fe(1)-O(1) bond lengths are 1.91 Å. In the second Fe site, Fe(2) is bonded to one O(2), one O(3), two equivalent O(1), and two equivalent O(4) atoms to form FeO6 octahedra that share corners with three equivalent Fe(1)O4 tetrahedra and edges with four equivalent Fe(2)O6 octahedra. The Fe(2)-O(2) bond length is 2.11 Å. The Fe(2)-O(3) bond length is 2.02 Å. Both Fe(2)-O(1) bond lengths are 1.97 Å. Both Fe(2)-O(4) bond lengths are 1.87 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Fe(1) and two equivalent Fe(2) atoms. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Fe(1) and three equivalent Fe(2) atoms. In the third O site, O(3) is bonded in a distorted T-shaped geometry to three equivalent Fe(2) atoms. In the fourth O site, O(4) is bonded in a water-like geometry to two equivalent Fe(2) atoms.