FeO2 crystallizes in the orthorhombic Imma space group. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-55°. In the second Fe site, Fe(2) is bonded to two equivalent O(1) and four equivalent O(2) atoms to form FeO6 octahedra that share corners with six equivalent Fe(1)O4 tetrahedra and edges with two equivalent Fe(2)O6 octahedra. There are two inequivalent O sites. In the first O site, O(2) is bonded in a trigonal planar geometry to one Fe(1) and two equivalent Fe(2) atoms. In the second O site, O(1) is bonded in a bent 120 degrees geometry to one Fe(1) and one Fe(2) atom.
FeO2 crystallizes in the orthorhombic Imma space group. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-55°. Both Fe(1)-O(1) bond lengths are 1.82 Å. Both Fe(1)-O(2) bond lengths are 1.88 Å. In the second Fe site, Fe(2) is bonded to two equivalent O(1) and four equivalent O(2) atoms to form FeO6 octahedra that share corners with six equivalent Fe(1)O4 tetrahedra and edges with two equivalent Fe(2)O6 octahedra. Both Fe(2)-O(1) bond lengths are 1.90 Å. All Fe(2)-O(2) bond lengths are 2.03 Å. There are two inequivalent O sites. In the first O site, O(2) is bonded in a trigonal planar geometry to one Fe(1) and two equivalent Fe(2) atoms. In the second O site, O(1) is bonded in a bent 120 degrees geometry to one Fe(1) and one Fe(2) atom.