The structure described by 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(6), and one O(7) atom to form FeO6 octahedra that share  a cornercorner with one Fe(9)O4 tetrahedra,  an edgeedge with one Fe(3)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(4)O6 octahedra. In the second Fe site, Fe(2) is bonded to one O(11), one O(12), one O(13), one O(4), one O(8), and one O(9) atom to form edge-sharing FeO6 octahedra. In the third Fe site, Fe(3) is bonded to one O(7), one O(8), two equivalent O(4), and two equivalent O(5) atoms to form edge-sharing FeO6 octahedra. In the fourth Fe site, Fe(4) is bonded to one O(3), one O(6), two equivalent O(1), and two equivalent O(5) atoms to form FeO6 octahedra that share  a cornercorner with one Fe(9)O4 tetrahedra,  an edgeedge with one Fe(3)O6 octahedra, and edges with four equivalent Fe(1)O6 octahedra. In the fifth Fe site, Fe(5) is bonded to one O(13), one O(15), two equivalent O(10), and two equivalent O(11) atoms to form edge-sharing FeO6 octahedra. In the sixth Fe site, Fe(6) is bonded to one O(12), one O(9), two equivalent O(11), and two equivalent O(4) atoms to form edge-sharing FeO6 octahedra. In the seventh Fe site, Fe(7) is bonded to one O(10), one O(14), one O(15), one O(16), one O(17), and one O(18) atom to form FeO6 octahedra that share corners with two equivalent Fe(9)O4 tetrahedra,  an edgeedge with one Fe(5)O6 octahedra, edges with two equivalent Fe(7)O6 octahedra, and edges with two equivalent Fe(8)O6 octahedra. In the eighth Fe site, Fe(8) is bonded to one O(14), one O(18), two equivalent O(10), and two equivalent O(16) atoms to form FeO6 octahedra that share corners with two equivalent Fe(9)O4 tetrahedra,  an edgeedge with one Fe(5)O6 octahedra, and edges with four equivalent Fe(7)O6 octahedra. In the ninth Fe site, Fe(9) is bonded to one O(17), one O(6), and two equivalent O(16) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 53-58°. 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(4) 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(4) and two equivalent Fe(1) atoms. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one Fe(2), one Fe(3), and one Fe(6) atom. In the fifth O site, O(5) is bonded in a trigonal non-coplanar geometry to one Fe(1), one Fe(3), and one Fe(4) atom. In the sixth O site, O(6) is bonded in a distorted rectangular see-saw-like geometry to one Fe(4), one Fe(9), and two equivalent Fe(1) atoms. In the seventh O site, O(7) is bonded in a trigonal non-coplanar geometry to one Fe(3) and two equivalent Fe(1) atoms. In the eighth O site, O(8) is bonded in a distorted trigonal non-coplanar geometry to one Fe(3) and two equivalent Fe(2) atoms. In the ninth O site, O(9) is bonded in a distorted trigonal non-coplanar geometry to one Fe(6) and two equivalent Fe(2) atoms. In the tenth O site, O(10) is bonded in a distorted T-shaped geometry to one Fe(5), one Fe(7), and one Fe(8) atom. In the eleventh O site, O(11) is bonded in a distorted T-shaped geometry to one Fe(2), one Fe(5), and one Fe(6) atom. In the twelfth O site, O(12) is bonded in a distorted trigonal non-coplanar geometry to one Fe(6) and two equivalent Fe(2) atoms. In the thirteenth O site, O(13) is bonded in a distorted trigonal non-coplanar geometry to one Fe(5) and two equivalent Fe(2) atoms. In the fourteenth O site, O(14) is bonded in a distorted trigonal non-coplanar geometry to one Fe(8) and two equivalent Fe(7) atoms. In the fifteenth O site, O(15) is bonded in a distorted T-shaped geometry to one Fe(5) and two equivalent Fe(7) atoms. In the sixteenth O site, O(16) is bonded in a distorted trigonal planar geometry to one Fe(7), one Fe(8), and one Fe(9) atom. In the seventeenth O site, O(17) is bonded in a trigonal planar geometry to one Fe(9) and two equivalent Fe(7) atoms. In the eighteenth O site, O(18) is bonded in a distorted trigonal non-coplanar geometry to one Fe(8) and two equivalent Fe(7) atoms. is represented by the Crystallographic Information File (CIF) [CIF]
data_FeO2
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   5.991
_cell_length_b   5.991
_cell_length_c   14.400
_cell_angle_alpha   89.667
_cell_angle_beta   89.667
_cell_angle_gamma   60.179
_symmetry_Int_Tables_number   1
_cell_volume   448.333
_cell_formula_units_Z   12
loop_
 _symmetry_equiv_pos_site_id
 _symmetry_equiv_pos_as_xyz
  1  'x, y, z'
loop_
 _atom_site_type_symbol
 _atom_site_label
 _atom_site_symmetry_multiplicity
 _atom_site_fract_x
 _atom_site_fract_y
 _atom_site_fract_z
 _atom_site_occupancy
  Fe  Fe0  1  0.678  0.160  0.833  1.0
  Fe  Fe1  1  0.500  0.999  0.500  1.0
  Fe  Fe2  1  0.334  0.334  0.666  1.0
  Fe  Fe3  1  0.161  0.161  0.832  1.0
  Fe  Fe4  1  0.999  0.500  0.500  1.0
  Fe  Fe5  1  0.160  0.678  0.833  1.0
  Fe  Fe6  1  0.664  0.664  0.333  1.0
  Fe  Fe7  1  0.499  0.499  0.500  1.0
  Fe  Fe8  1  0.333  0.832  0.166  1.0
  Fe  Fe9  1  0.834  0.834  0.166  1.0
  Fe  Fe10  1  0.832  0.333  0.166  1.0
  Fe  Fe11  1  0.335  0.335  0.046  1.0
  O  O12  1  0.305  0.845  0.894  1.0
  O  O13  1  0.847  0.847  0.895  1.0
  O  O14  1  0.998  0.998  0.756  1.0
  O  O15  1  0.845  0.305  0.894  1.0
  O  O16  1  0.628  0.197  0.579  1.0
  O  O17  1  0.473  0.054  0.757  1.0
  O  O18  1  0.334  0.334  0.912  1.0
  O  O19  1  0.473  0.473  0.756  1.0
  O  O20  1  0.191  0.191  0.583  1.0
  O  O21  1  0.338  0.338  0.432  1.0
  O  O22  1  0.197  0.628  0.579  1.0
  O  O23  1  0.054  0.473  0.757  1.0
  O  O24  1  0.945  0.524  0.254  1.0
  O  O25  1  0.801  0.376  0.419  1.0
  O  O26  1  0.662  0.662  0.568  1.0
  O  O27  1  0.807  0.807  0.417  1.0
  O  O28  1  0.665  0.665  0.099  1.0
  O  O29  1  0.522  0.522  0.253  1.0
  O  O30  1  0.524  0.945  0.254  1.0
  O  O31  1  0.376  0.801  0.419  1.0
  O  O32  1  0.170  0.664  0.098  1.0
  O  O33  1  0.169  0.169  0.098  1.0
  O  O34  1  0.001  0.001  0.231  1.0
  O  O35  1  0.664  0.170  0.098  1.0
[/CIF]
.
The structure described by 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(6), and one O(7) atom to form FeO6 octahedra that share  a cornercorner with one Fe(9)O4 tetrahedra,  an edgeedge with one Fe(3)O6 octahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(4)O6 octahedra. In the second Fe site, Fe(2) is bonded to one O(11), one O(12), one O(13), one O(4), one O(8), and one O(9) atom to form edge-sharing FeO6 octahedra. In the third Fe site, Fe(3) is bonded to one O(7), one O(8), two equivalent O(4), and two equivalent O(5) atoms to form edge-sharing FeO6 octahedra. In the fourth Fe site, Fe(4) is bonded to one O(3), one O(6), two equivalent O(1), and two equivalent O(5) atoms to form FeO6 octahedra that share  a cornercorner with one Fe(9)O4 tetrahedra,  an edgeedge with one Fe(3)O6 octahedra, and edges with four equivalent Fe(1)O6 octahedra. In the fifth Fe site, Fe(5) is bonded to one O(13), one O(15), two equivalent O(10), and two equivalent O(11) atoms to form edge-sharing FeO6 octahedra. In the sixth Fe site, Fe(6) is bonded to one O(12), one O(9), two equivalent O(11), and two equivalent O(4) atoms to form edge-sharing FeO6 octahedra. In the seventh Fe site, Fe(7) is bonded to one O(10), one O(14), one O(15), one O(16), one O(17), and one O(18) atom to form FeO6 octahedra that share corners with two equivalent Fe(9)O4 tetrahedra,  an edgeedge with one Fe(5)O6 octahedra, edges with two equivalent Fe(7)O6 octahedra, and edges with two equivalent Fe(8)O6 octahedra. In the eighth Fe site, Fe(8) is bonded to one O(14), one O(18), two equivalent O(10), and two equivalent O(16) atoms to form FeO6 octahedra that share corners with two equivalent Fe(9)O4 tetrahedra,  an edgeedge with one Fe(5)O6 octahedra, and edges with four equivalent Fe(7)O6 octahedra. In the ninth Fe site, Fe(9) is bonded to one O(17), one O(6), and two equivalent O(16) atoms to form corner-sharing FeO4 tetrahedra. The corner-sharing octahedral tilt angles range from 53-58°. 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(4) 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(4) and two equivalent Fe(1) atoms. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one Fe(2), one Fe(3), and one Fe(6) atom. In the fifth O site, O(5) is bonded in a trigonal non-coplanar geometry to one Fe(1), one Fe(3), and one Fe(4) atom. In the sixth O site, O(6) is bonded in a distorted rectangular see-saw-like geometry to one Fe(4), one Fe(9), and two equivalent Fe(1) atoms. In the seventh O site, O(7) is bonded in a trigonal non-coplanar geometry to one Fe(3) and two equivalent Fe(1) atoms. In the eighth O site, O(8) is bonded in a distorted trigonal non-coplanar geometry to one Fe(3) and two equivalent Fe(2) atoms. In the ninth O site, O(9) is bonded in a distorted trigonal non-coplanar geometry to one Fe(6) and two equivalent Fe(2) atoms. In the tenth O site, O(10) is bonded in a distorted T-shaped geometry to one Fe(5), one Fe(7), and one Fe(8) atom. In the eleventh O site, O(11) is bonded in a distorted T-shaped geometry to one Fe(2), one Fe(5), and one Fe(6) atom. In the twelfth O site, O(12) is bonded in a distorted trigonal non-coplanar geometry to one Fe(6) and two equivalent Fe(2) atoms. In the thirteenth O site, O(13) is bonded in a distorted trigonal non-coplanar geometry to one Fe(5) and two equivalent Fe(2) atoms. In the fourteenth O site, O(14) is bonded in a distorted trigonal non-coplanar geometry to one Fe(8) and two equivalent Fe(7) atoms. In the fifteenth O site, O(15) is bonded in a distorted T-shaped geometry to one Fe(5) and two equivalent Fe(7) atoms. In the sixteenth O site, O(16) is bonded in a distorted trigonal planar geometry to one Fe(7), one Fe(8), and one Fe(9) atom. In the seventeenth O site, O(17) is bonded in a trigonal planar geometry to one Fe(9) and two equivalent Fe(7) atoms. In the eighteenth O site, O(18) is bonded in a distorted trigonal non-coplanar geometry to one Fe(8) and two equivalent Fe(7) atoms. is represented by the Crystallographic Information File (CIF) [CIF]
data_FeO2
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   5.991
_cell_length_b   5.991
_cell_length_c   14.400
_cell_angle_alpha   89.667
_cell_angle_beta   89.667
_cell_angle_gamma   60.179
_symmetry_Int_Tables_number   1
_cell_volume   448.333
_cell_formula_units_Z   12
loop_
 _symmetry_equiv_pos_site_id
 _symmetry_equiv_pos_as_xyz
  1  'x, y, z'
loop_
 _atom_site_type_symbol
 _atom_site_label
 _atom_site_symmetry_multiplicity
 _atom_site_fract_x
 _atom_site_fract_y
 _atom_site_fract_z
 _atom_site_occupancy
  Fe  Fe0  1  0.678  0.160  0.833  1.0
  Fe  Fe1  1  0.500  0.999  0.500  1.0
  Fe  Fe2  1  0.334  0.334  0.666  1.0
  Fe  Fe3  1  0.161  0.161  0.832  1.0
  Fe  Fe4  1  0.999  0.500  0.500  1.0
  Fe  Fe5  1  0.160  0.678  0.833  1.0
  Fe  Fe6  1  0.664  0.664  0.333  1.0
  Fe  Fe7  1  0.499  0.499  0.500  1.0
  Fe  Fe8  1  0.333  0.832  0.166  1.0
  Fe  Fe9  1  0.834  0.834  0.166  1.0
  Fe  Fe10  1  0.832  0.333  0.166  1.0
  Fe  Fe11  1  0.335  0.335  0.046  1.0
  O  O12  1  0.305  0.845  0.894  1.0
  O  O13  1  0.847  0.847  0.895  1.0
  O  O14  1  0.998  0.998  0.756  1.0
  O  O15  1  0.845  0.305  0.894  1.0
  O  O16  1  0.628  0.197  0.579  1.0
  O  O17  1  0.473  0.054  0.757  1.0
  O  O18  1  0.334  0.334  0.912  1.0
  O  O19  1  0.473  0.473  0.756  1.0
  O  O20  1  0.191  0.191  0.583  1.0
  O  O21  1  0.338  0.338  0.432  1.0
  O  O22  1  0.197  0.628  0.579  1.0
  O  O23  1  0.054  0.473  0.757  1.0
  O  O24  1  0.945  0.524  0.254  1.0
  O  O25  1  0.801  0.376  0.419  1.0
  O  O26  1  0.662  0.662  0.568  1.0
  O  O27  1  0.807  0.807  0.417  1.0
  O  O28  1  0.665  0.665  0.099  1.0
  O  O29  1  0.522  0.522  0.253  1.0
  O  O30  1  0.524  0.945  0.254  1.0
  O  O31  1  0.376  0.801  0.419  1.0
  O  O32  1  0.170  0.664  0.098  1.0
  O  O33  1  0.169  0.169  0.098  1.0
  O  O34  1  0.001  0.001  0.231  1.0
  O  O35  1  0.664  0.170  0.098  1.0
[/CIF]
.