FeAsO4 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(2) and four equivalent O(4) atoms to form FeO6 octahedra that share corners with four equivalent Fe(2)O6 octahedra, corners with two equivalent As(1)O4 tetrahedra, and corners with four equivalent As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 53°. In the second Fe site, Fe(2) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form FeO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent As(1)O4 tetrahedra, corners with two equivalent As(2)O4 tetrahedra,  an edgeedge with one Fe(2)O6 octahedra, and  an edgeedge with one As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 53°. There are two inequivalent As sites. In the first As site, As(1) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form AsO4 tetrahedra that share corners with two equivalent Fe(1)O6 octahedra and corners with four equivalent Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 43-55°. In the second As site, As(2) is bonded to two equivalent O(3) and two equivalent O(4) atoms to form AsO4 tetrahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent Fe(2)O6 octahedra, and  an edgeedge with one Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 41-57°. There are four inequivalent O sites. In the first O site, O(3) is bonded in a bent 150 degrees geometry to one Fe(2) and one As(2) atom. In the second O site, O(4) is bonded in a distorted trigonal planar geometry to one Fe(1), one Fe(2), and one As(2) atom. In the third O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Fe(2) and one As(1) atom. In the fourth O site, O(2) is bonded in a bent 150 degrees geometry to one Fe(1) and one As(1) atom.
FeAsO4 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(2) and four equivalent O(4) atoms to form FeO6 octahedra that share corners with four equivalent Fe(2)O6 octahedra, corners with two equivalent As(1)O4 tetrahedra, and corners with four equivalent As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 53°. Both Fe(1)-O(2) bond lengths are 1.99 Å. All Fe(1)-O(4) bond lengths are 2.03 Å. In the second Fe site, Fe(2) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form FeO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent As(1)O4 tetrahedra, corners with two equivalent As(2)O4 tetrahedra,  an edgeedge with one Fe(2)O6 octahedra, and  an edgeedge with one As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 53°. Both Fe(2)-O(1) bond lengths are 2.03 Å. Both Fe(2)-O(3) bond lengths are 1.95 Å. Both Fe(2)-O(4) bond lengths are 2.08 Å. There are two inequivalent As sites. In the first As site, As(1) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form AsO4 tetrahedra that share corners with two equivalent Fe(1)O6 octahedra and corners with four equivalent Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 43-55°. Both As(1)-O(1) bond lengths are 1.72 Å. Both As(1)-O(2) bond lengths are 1.68 Å. In the second As site, As(2) is bonded to two equivalent O(3) and two equivalent O(4) atoms to form AsO4 tetrahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent Fe(2)O6 octahedra, and  an edgeedge with one Fe(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 41-57°. Both As(2)-O(3) bond lengths are 1.67 Å. Both As(2)-O(4) bond lengths are 1.74 Å. There are four inequivalent O sites. In the first O site, O(3) is bonded in a bent 150 degrees geometry to one Fe(2) and one As(2) atom. In the second O site, O(4) is bonded in a distorted trigonal planar geometry to one Fe(1), one Fe(2), and one As(2) atom. In the third O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Fe(2) and one As(1) atom. In the fourth O site, O(2) is bonded in a bent 150 degrees geometry to one Fe(1) and one As(1) atom.