FeO2 crystallizes in the monoclinic Cm space group. There are nine inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(7), and one O(8) atom to form FeO6 octahedra that share  a cornercorner with one Fe(2)O4 tetrahedra,  a cornercorner with one Fe(9)O4 tetrahedra, corners with two equivalent Fe(7)O4 tetrahedra,  an edgeedge with one Fe(3)O6 octahedra, and edges with two equivalent Fe(1)O6 octahedra. In the second Fe site, Fe(2) is bonded to one O(2), one O(9), and two equivalent O(4) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-57°. In the third Fe site, Fe(3) is bonded to one O(16), one O(3), two equivalent O(1), and two equivalent O(17) atoms to form FeO6 octahedra that share corners with three equivalent Fe(9)O4 tetrahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(6)O6 octahedra. In the fourth Fe site, Fe(4) is bonded to one O(11), one O(12), one O(14), one O(4), one O(6), and one O(9) atom to form FeO6 octahedra that share  a cornercorner with one Fe(5)O4 tetrahedra,  a cornercorner with one Fe(7)O4 tetrahedra, corners with two equivalent Fe(2)O4 tetrahedra, corners with two equivalent Fe(8)O4 tetrahedra, and edges with two equivalent Fe(4)O6 octahedra. In the fifth Fe site, Fe(5) is bonded to one O(15), one O(6), and two equivalent O(10) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 48-56°. In the sixth Fe site, Fe(6) is bonded to one O(10), one O(13), one O(15), one O(16), one O(17), and one O(18) atom to form FeO6 octahedra that share  a cornercorner with one Fe(8)O4 tetrahedra, corners with two equivalent Fe(5)O4 tetrahedra, corners with two equivalent Fe(9)O4 tetrahedra,  an edgeedge with one Fe(3)O6 octahedra, and edges with two equivalent Fe(6)O6 octahedra. In the seventh Fe site, Fe(7) is bonded to one O(14), one O(5), and two equivalent O(7) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 52-55°. In the eighth Fe site, Fe(8) is bonded to one O(11), one O(18), and two equivalent O(12) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 48-57°. In the ninth Fe site, Fe(9) is bonded to one O(16), one O(8), and two equivalent O(17) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 54-59°. There are eighteen inequivalent O sites. In the first O site, O(1) is bonded in a water-like geometry to one Fe(1) and one Fe(3) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Fe(2) and two equivalent Fe(1) atoms. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Fe(3) and two equivalent Fe(1) atoms. In the fourth O site, O(4) is bonded in a bent 120 degrees geometry to one Fe(2) and one Fe(4) atom. In the fifth O site, O(5) is bonded in a trigonal planar geometry to one Fe(7) and two equivalent Fe(1) atoms. In the sixth O site, O(6) is bonded in a trigonal planar geometry to one Fe(5) and two equivalent Fe(4) atoms. In the seventh O site, O(7) is bonded in a bent 120 degrees geometry to one Fe(1) and one Fe(7) atom. In the eighth O site, O(8) is bonded in a trigonal planar geometry to one Fe(9) and two equivalent Fe(1) atoms. In the ninth O site, O(9) is bonded in a trigonal planar geometry to one Fe(2) and two equivalent Fe(4) atoms. In the tenth O site, O(10) is bonded in a bent 120 degrees geometry to one Fe(5) and one Fe(6) atom. In the eleventh O site, O(11) is bonded in a distorted trigonal planar geometry to one Fe(8) and two equivalent Fe(4) atoms. In the twelfth O site, O(12) is bonded in a bent 120 degrees geometry to one Fe(4) and one Fe(8) atom. In the thirteenth O site, O(13) is bonded in a water-like geometry to two equivalent Fe(6) atoms. In the fourteenth O site, O(14) is bonded in a trigonal planar geometry to one Fe(7) and two equivalent Fe(4) atoms. In the fifteenth O site, O(15) is bonded in a distorted trigonal planar geometry to one Fe(5) and two equivalent Fe(6) atoms. In the sixteenth O site, O(16) is bonded in a rectangular see-saw-like geometry to one Fe(3), one Fe(9), and two equivalent Fe(6) atoms. In the seventeenth O site, O(17) is bonded in a trigonal planar geometry to one Fe(3), one Fe(6), and one Fe(9) atom. In the eighteenth O site, O(18) is bonded in a trigonal planar geometry to one Fe(8) and two equivalent Fe(6) atoms.
FeO2 crystallizes in the monoclinic Cm space group. There are nine inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(7), and one O(8) atom to form FeO6 octahedra that share  a cornercorner with one Fe(2)O4 tetrahedra,  a cornercorner with one Fe(9)O4 tetrahedra, corners with two equivalent Fe(7)O4 tetrahedra,  an edgeedge with one Fe(3)O6 octahedra, and edges with two equivalent Fe(1)O6 octahedra. The Fe(1)-O(1) bond length is 1.88 Å. The Fe(1)-O(2) bond length is 2.04 Å. The Fe(1)-O(3) bond length is 2.05 Å. The Fe(1)-O(5) bond length is 1.99 Å. The Fe(1)-O(7) bond length is 1.85 Å. The Fe(1)-O(8) bond length is 2.00 Å. In the second Fe site, Fe(2) is bonded to one O(2), one O(9), and two equivalent O(4) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-57°. The Fe(2)-O(2) bond length is 1.85 Å. The Fe(2)-O(9) bond length is 1.87 Å. Both Fe(2)-O(4) bond lengths are 1.79 Å. In the third Fe site, Fe(3) is bonded to one O(16), one O(3), two equivalent O(1), and two equivalent O(17) atoms to form FeO6 octahedra that share corners with three equivalent Fe(9)O4 tetrahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(6)O6 octahedra. The Fe(3)-O(16) bond length is 1.97 Å. The Fe(3)-O(3) bond length is 1.89 Å. Both Fe(3)-O(1) bond lengths are 1.85 Å. Both Fe(3)-O(17) bond lengths are 1.98 Å. In the fourth Fe site, Fe(4) is bonded to one O(11), one O(12), one O(14), one O(4), one O(6), and one O(9) atom to form FeO6 octahedra that share  a cornercorner with one Fe(5)O4 tetrahedra,  a cornercorner with one Fe(7)O4 tetrahedra, corners with two equivalent Fe(2)O4 tetrahedra, corners with two equivalent Fe(8)O4 tetrahedra, and edges with two equivalent Fe(4)O6 octahedra. The Fe(4)-O(11) bond length is 2.03 Å. The Fe(4)-O(12) bond length is 1.95 Å. The Fe(4)-O(14) bond length is 2.01 Å. The Fe(4)-O(4) bond length is 1.96 Å. The Fe(4)-O(6) bond length is 2.03 Å. The Fe(4)-O(9) bond length is 2.02 Å. In the fifth Fe site, Fe(5) is bonded to one O(15), one O(6), and two equivalent O(10) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 48-56°. The Fe(5)-O(15) bond length is 1.87 Å. The Fe(5)-O(6) bond length is 1.86 Å. Both Fe(5)-O(10) bond lengths are 1.78 Å. In the sixth Fe site, Fe(6) is bonded to one O(10), one O(13), one O(15), one O(16), one O(17), and one O(18) atom to form FeO6 octahedra that share  a cornercorner with one Fe(8)O4 tetrahedra, corners with two equivalent Fe(5)O4 tetrahedra, corners with two equivalent Fe(9)O4 tetrahedra,  an edgeedge with one Fe(3)O6 octahedra, and edges with two equivalent Fe(6)O6 octahedra. The Fe(6)-O(10) bond length is 1.91 Å. The Fe(6)-O(13) bond length is 1.88 Å. The Fe(6)-O(15) bond length is 2.01 Å. The Fe(6)-O(16) bond length is 2.17 Å. The Fe(6)-O(17) bond length is 1.97 Å. The Fe(6)-O(18) bond length is 1.97 Å. In the seventh Fe site, Fe(7) is bonded to one O(14), one O(5), and two equivalent O(7) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 52-55°. The Fe(7)-O(14) bond length is 1.90 Å. The Fe(7)-O(5) bond length is 1.89 Å. Both Fe(7)-O(7) bond lengths are 1.83 Å. In the eighth Fe site, Fe(8) is bonded to one O(11), one O(18), and two equivalent O(12) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 48-57°. The Fe(8)-O(11) bond length is 1.89 Å. The Fe(8)-O(18) bond length is 1.94 Å. Both Fe(8)-O(12) bond lengths are 1.80 Å. In the ninth Fe site, Fe(9) is bonded to one O(16), one O(8), and two equivalent O(17) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 54-59°. The Fe(9)-O(16) bond length is 1.97 Å. The Fe(9)-O(8) bond length is 1.88 Å. Both Fe(9)-O(17) bond lengths are 1.92 Å. There are eighteen inequivalent O sites. In the first O site, O(1) is bonded in a water-like geometry to one Fe(1) and one Fe(3) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Fe(2) and two equivalent Fe(1) atoms. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Fe(3) and two equivalent Fe(1) atoms. In the fourth O site, O(4) is bonded in a bent 120 degrees geometry to one Fe(2) and one Fe(4) atom. In the fifth O site, O(5) is bonded in a trigonal planar geometry to one Fe(7) and two equivalent Fe(1) atoms. In the sixth O site, O(6) is bonded in a trigonal planar geometry to one Fe(5) and two equivalent Fe(4) atoms. In the seventh O site, O(7) is bonded in a bent 120 degrees geometry to one Fe(1) and one Fe(7) atom. In the eighth O site, O(8) is bonded in a trigonal planar geometry to one Fe(9) and two equivalent Fe(1) atoms. In the ninth O site, O(9) is bonded in a trigonal planar geometry to one Fe(2) and two equivalent Fe(4) atoms. In the tenth O site, O(10) is bonded in a bent 120 degrees geometry to one Fe(5) and one Fe(6) atom. In the eleventh O site, O(11) is bonded in a distorted trigonal planar geometry to one Fe(8) and two equivalent Fe(4) atoms. In the twelfth O site, O(12) is bonded in a bent 120 degrees geometry to one Fe(4) and one Fe(8) atom. In the thirteenth O site, O(13) is bonded in a water-like geometry to two equivalent Fe(6) atoms. In the fourteenth O site, O(14) is bonded in a trigonal planar geometry to one Fe(7) and two equivalent Fe(4) atoms. In the fifteenth O site, O(15) is bonded in a distorted trigonal planar geometry to one Fe(5) and two equivalent Fe(6) atoms. In the sixteenth O site, O(16) is bonded in a rectangular see-saw-like geometry to one Fe(3), one Fe(9), and two equivalent Fe(6) atoms. In the seventeenth O site, O(17) is bonded in a trigonal planar geometry to one Fe(3), one Fe(6), and one Fe(9) atom. In the eighteenth O site, O(18) is bonded in a trigonal planar geometry to one Fe(8) and two equivalent Fe(6) atoms.