FeO2 crystallizes in the monoclinic Cm space group. There are twelve inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(6), and one O(8) atom to form FeO6 octahedra that share  a cornercorner with one Fe(12)O4 tetrahedra, corners with two equivalent Fe(5)O4 tetrahedra,  an edgeedge with one Fe(11)O6 octahedra,  an edgeedge with one Fe(2)O6 octahedra, and edges with two equivalent Fe(1)O6 octahedra. In the second Fe site, Fe(2) is bonded to one O(4), one O(9), two equivalent O(5), and two equivalent O(6) atoms to form FeO6 octahedra that share corners with three equivalent Fe(5)O4 tetrahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(4)O6 octahedra. In the third Fe site, Fe(3) is bonded to one O(15), one O(7), and two equivalent O(10) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 52-56°. In the fourth Fe site, Fe(4) is bonded to one O(11), one O(12), one O(14), one O(5), one O(7), and one O(9) atom to form FeO6 octahedra that share  a cornercorner with one Fe(3)O4 tetrahedra,  a cornercorner with one Fe(5)O4 tetrahedra, corners with two equivalent Fe(8)O4 tetrahedra,  an edgeedge with one Fe(2)O6 octahedra, and edges with two equivalent Fe(4)O6 octahedra. In the fifth Fe site, Fe(5) is bonded to one O(14), one O(4), and two equivalent O(6) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 56-60°. In the sixth Fe site, Fe(6) is bonded to one O(13), one O(21), and two equivalent O(17) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-56°. In the seventh Fe site, Fe(7) is bonded to one O(10), one O(13), one O(15), one O(16), one O(18), and one O(20) atom to form FeO6 octahedra that share  a cornercorner with one Fe(6)O4 tetrahedra,  a cornercorner with one Fe(8)O4 tetrahedra, corners with two equivalent Fe(10)O4 tetrahedra, corners with two equivalent Fe(3)O4 tetrahedra, and edges with two equivalent Fe(7)O6 octahedra. In the eighth Fe site, Fe(8) is bonded to one O(11), one O(20), and two equivalent O(12) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 52-55°. In the ninth Fe site, Fe(9) is bonded to one O(17), one O(19), one O(21), one O(22), one O(23), and one O(24) atom to form FeO6 octahedra that share  a cornercorner with one Fe(10)O4 tetrahedra, corners with two equivalent Fe(12)O4 tetrahedra, corners with two equivalent Fe(6)O4 tetrahedra,  an edgeedge with one Fe(11)O6 octahedra, and edges with two equivalent Fe(9)O6 octahedra. In the tenth Fe site, Fe(10) is bonded to one O(16), one O(24), and two equivalent O(18) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-57°. In the eleventh Fe site, Fe(11) is bonded to one O(22), one O(3), two equivalent O(1), and two equivalent O(23) atoms to form FeO6 octahedra that share corners with three equivalent Fe(12)O4 tetrahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(9)O6 octahedra. In the twelfth Fe site, Fe(12) is bonded to one O(22), one O(8), and two equivalent O(23) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 56-58°. There are twenty-four inequivalent O sites. In the first O site, O(1) is bonded in a water-like geometry to one Fe(1) and one Fe(11) atom. In the second O site, O(2) is bonded in a water-like geometry to two equivalent Fe(1) atoms. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Fe(11) and two equivalent Fe(1) atoms. In the fourth O site, O(4) is bonded in a distorted rectangular see-saw-like geometry to one Fe(2), one Fe(5), and two equivalent Fe(1) atoms. In the fifth O site, O(5) is bonded in a water-like geometry to one Fe(2) and one Fe(4) atom. In the sixth O site, O(6) is bonded in a trigonal planar geometry to one Fe(1), one Fe(2), and one Fe(5) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to one Fe(3) and two equivalent Fe(4) atoms. In the eighth O site, O(8) is bonded in a trigonal planar geometry to one Fe(12) and two equivalent Fe(1) atoms. In the ninth O site, O(9) is bonded in a distorted T-shaped 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(3) and one Fe(7) atom. In the eleventh O site, O(11) is bonded in a 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 distorted trigonal planar geometry to one Fe(6) and two equivalent Fe(7) atoms. In the fourteenth O site, O(14) is bonded in a distorted trigonal planar geometry to one Fe(5) and two equivalent Fe(4) atoms. In the fifteenth O site, O(15) is bonded in a trigonal planar geometry to one Fe(3) and two equivalent Fe(7) atoms. In the sixteenth O site, O(16) is bonded in a distorted trigonal planar geometry to one Fe(10) and two equivalent Fe(7) atoms. In the seventeenth O site, O(17) is bonded in a bent 120 degrees geometry to one Fe(6) and one Fe(9) atom. In the eighteenth O site, O(18) is bonded in a bent 120 degrees geometry to one Fe(10) and one Fe(7) atom. In the nineteenth O site, O(19) is bonded in a water-like geometry to two equivalent Fe(9) atoms. In the twentieth O site, O(20) is bonded in a distorted trigonal planar geometry to one Fe(8) and two equivalent Fe(7) atoms. In the twenty-first O site, O(21) is bonded in a distorted trigonal planar geometry to one Fe(6) and two equivalent Fe(9) atoms. In the twenty-second O site, O(22) is bonded in a distorted rectangular see-saw-like geometry to one Fe(11), one Fe(12), and two equivalent Fe(9) atoms. In the twenty-third O site, O(23) is bonded in a trigonal planar geometry to one Fe(11), one Fe(12), and one Fe(9) atom. In the twenty-fourth O site, O(24) is bonded in a trigonal planar geometry to one Fe(10) and two equivalent Fe(9) atoms.
FeO2 crystallizes in the monoclinic Cm space group. There are twelve inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(6), and one O(8) atom to form FeO6 octahedra that share  a cornercorner with one Fe(12)O4 tetrahedra, corners with two equivalent Fe(5)O4 tetrahedra,  an edgeedge with one Fe(11)O6 octahedra,  an edgeedge with one Fe(2)O6 octahedra, and edges with two equivalent Fe(1)O6 octahedra. The Fe(1)-O(1) bond length is 1.85 Å. The Fe(1)-O(2) bond length is 1.88 Å. The Fe(1)-O(3) bond length is 2.00 Å. The Fe(1)-O(4) bond length is 2.11 Å. The Fe(1)-O(6) bond length is 1.96 Å. The Fe(1)-O(8) bond length is 1.98 Å. In the second Fe site, Fe(2) is bonded to one O(4), one O(9), two equivalent O(5), and two equivalent O(6) atoms to form FeO6 octahedra that share corners with three equivalent Fe(5)O4 tetrahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(4)O6 octahedra. The Fe(2)-O(4) bond length is 2.06 Å. The Fe(2)-O(9) bond length is 1.94 Å. Both Fe(2)-O(5) bond lengths are 1.86 Å. Both Fe(2)-O(6) bond lengths are 1.98 Å. In the third Fe site, Fe(3) is bonded to one O(15), one O(7), and two equivalent O(10) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 52-56°. The Fe(3)-O(15) bond length is 1.89 Å. The Fe(3)-O(7) bond length is 1.87 Å. Both Fe(3)-O(10) bond lengths are 1.82 Å. In the fourth Fe site, Fe(4) is bonded to one O(11), one O(12), one O(14), one O(5), one O(7), and one O(9) atom to form FeO6 octahedra that share  a cornercorner with one Fe(3)O4 tetrahedra,  a cornercorner with one Fe(5)O4 tetrahedra, corners with two equivalent Fe(8)O4 tetrahedra,  an edgeedge with one Fe(2)O6 octahedra, and edges with two equivalent Fe(4)O6 octahedra. The Fe(4)-O(11) bond length is 1.97 Å. The Fe(4)-O(12) bond length is 1.85 Å. The Fe(4)-O(14) bond length is 2.01 Å. The Fe(4)-O(5) bond length is 1.89 Å. The Fe(4)-O(7) bond length is 2.02 Å. The Fe(4)-O(9) bond length is 2.00 Å. In the fifth Fe site, Fe(5) is bonded to one O(14), one O(4), and two equivalent O(6) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 56-60°. The Fe(5)-O(14) bond length is 1.88 Å. The Fe(5)-O(4) bond length is 1.96 Å. Both Fe(5)-O(6) bond lengths are 1.91 Å. In the sixth Fe site, Fe(6) is bonded to one O(13), one O(21), and two equivalent O(17) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-56°. The Fe(6)-O(13) bond length is 1.86 Å. The Fe(6)-O(21) bond length is 1.85 Å. Both Fe(6)-O(17) bond lengths are 1.77 Å. In the seventh Fe site, Fe(7) is bonded to one O(10), one O(13), one O(15), one O(16), one O(18), and one O(20) atom to form FeO6 octahedra that share  a cornercorner with one Fe(6)O4 tetrahedra,  a cornercorner with one Fe(8)O4 tetrahedra, corners with two equivalent Fe(10)O4 tetrahedra, corners with two equivalent Fe(3)O4 tetrahedra, and edges with two equivalent Fe(7)O6 octahedra. The Fe(7)-O(10) bond length is 1.91 Å. The Fe(7)-O(13) bond length is 2.02 Å. The Fe(7)-O(15) bond length is 2.00 Å. The Fe(7)-O(16) bond length is 2.01 Å. The Fe(7)-O(18) bond length is 1.91 Å. The Fe(7)-O(20) bond length is 2.01 Å. In the eighth Fe site, Fe(8) is bonded to one O(11), one O(20), and two equivalent O(12) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 52-55°. The Fe(8)-O(11) bond length is 1.88 Å. The Fe(8)-O(20) bond length is 1.90 Å. Both Fe(8)-O(12) bond lengths are 1.83 Å. In the ninth Fe site, Fe(9) is bonded to one O(17), one O(19), one O(21), one O(22), one O(23), and one O(24) atom to form FeO6 octahedra that share  a cornercorner with one Fe(10)O4 tetrahedra, corners with two equivalent Fe(12)O4 tetrahedra, corners with two equivalent Fe(6)O4 tetrahedra,  an edgeedge with one Fe(11)O6 octahedra, and edges with two equivalent Fe(9)O6 octahedra. The Fe(9)-O(17) bond length is 1.91 Å. The Fe(9)-O(19) bond length is 1.86 Å. The Fe(9)-O(21) bond length is 2.00 Å. The Fe(9)-O(22) bond length is 2.14 Å. The Fe(9)-O(23) bond length is 2.01 Å. The Fe(9)-O(24) bond length is 1.96 Å. In the tenth Fe site, Fe(10) is bonded to one O(16), one O(24), and two equivalent O(18) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-57°. The Fe(10)-O(16) bond length is 1.89 Å. The Fe(10)-O(24) bond length is 1.93 Å. Both Fe(10)-O(18) bond lengths are 1.82 Å. In the eleventh Fe site, Fe(11) is bonded to one O(22), one O(3), two equivalent O(1), and two equivalent O(23) atoms to form FeO6 octahedra that share corners with three equivalent Fe(12)O4 tetrahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(9)O6 octahedra. The Fe(11)-O(22) bond length is 2.06 Å. The Fe(11)-O(3) bond length is 2.02 Å. Both Fe(11)-O(1) bond lengths are 1.87 Å. Both Fe(11)-O(23) bond lengths are 1.96 Å. In the twelfth Fe site, Fe(12) is bonded to one O(22), one O(8), and two equivalent O(23) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 56-58°. The Fe(12)-O(22) bond length is 1.96 Å. The Fe(12)-O(8) bond length is 1.91 Å. Both Fe(12)-O(23) bond lengths are 1.92 Å. There are twenty-four inequivalent O sites. In the first O site, O(1) is bonded in a water-like geometry to one Fe(1) and one Fe(11) atom. In the second O site, O(2) is bonded in a water-like geometry to two equivalent Fe(1) atoms. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Fe(11) and two equivalent Fe(1) atoms. In the fourth O site, O(4) is bonded in a distorted rectangular see-saw-like geometry to one Fe(2), one Fe(5), and two equivalent Fe(1) atoms. In the fifth O site, O(5) is bonded in a water-like geometry to one Fe(2) and one Fe(4) atom. In the sixth O site, O(6) is bonded in a trigonal planar geometry to one Fe(1), one Fe(2), and one Fe(5) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to one Fe(3) and two equivalent Fe(4) atoms. In the eighth O site, O(8) is bonded in a trigonal planar geometry to one Fe(12) and two equivalent Fe(1) atoms. In the ninth O site, O(9) is bonded in a distorted T-shaped 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(3) and one Fe(7) atom. In the eleventh O site, O(11) is bonded in a 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 distorted trigonal planar geometry to one Fe(6) and two equivalent Fe(7) atoms. In the fourteenth O site, O(14) is bonded in a distorted trigonal planar geometry to one Fe(5) and two equivalent Fe(4) atoms. In the fifteenth O site, O(15) is bonded in a trigonal planar geometry to one Fe(3) and two equivalent Fe(7) atoms. In the sixteenth O site, O(16) is bonded in a distorted trigonal planar geometry to one Fe(10) and two equivalent Fe(7) atoms. In the seventeenth O site, O(17) is bonded in a bent 120 degrees geometry to one Fe(6) and one Fe(9) atom. In the eighteenth O site, O(18) is bonded in a bent 120 degrees geometry to one Fe(10) and one Fe(7) atom. In the nineteenth O site, O(19) is bonded in a water-like geometry to two equivalent Fe(9) atoms. In the twentieth O site, O(20) is bonded in a distorted trigonal planar geometry to one Fe(8) and two equivalent Fe(7) atoms. In the twenty-first O site, O(21) is bonded in a distorted trigonal planar geometry to one Fe(6) and two equivalent Fe(9) atoms. In the twenty-second O site, O(22) is bonded in a distorted rectangular see-saw-like geometry to one Fe(11), one Fe(12), and two equivalent Fe(9) atoms. In the twenty-third O site, O(23) is bonded in a trigonal planar geometry to one Fe(11), one Fe(12), and one Fe(9) atom. In the twenty-fourth O site, O(24) is bonded in a trigonal planar geometry to one Fe(10) and two equivalent Fe(9) atoms.