Ni2As2O9 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(1), one O(2), one O(3), one O(6), one O(7), and one O(8) atom to form NiO6 octahedra that share corners with two equivalent As(2)O4 tetrahedra, corners with three equivalent As(1)O4 tetrahedra, and edges with two equivalent Ni(2)O6 octahedra. In the second Ni site, Ni(2) is bonded to one O(1), one O(3), one O(5), one O(6), one O(7), and one O(9) atom to form NiO6 octahedra that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra, and edges with two equivalent Ni(1)O6 octahedra. There are two inequivalent As sites. In the first As site, As(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form AsO4 tetrahedra that share corners with two equivalent Ni(2)O6 octahedra, corners with three equivalent Ni(1)O6 octahedra, and  a cornercorner with one As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-62°. In the second As site, As(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form AsO4 tetrahedra that share corners with two equivalent Ni(1)O6 octahedra, corners with three equivalent Ni(2)O6 octahedra, and  a cornercorner with one As(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 47-62°. There are nine inequivalent O sites. In the first O site, O(5) is bonded in a bent 120 degrees geometry to one Ni(2) and one As(2) atom. In the second O site, O(6) is bonded in a distorted trigonal planar geometry to one Ni(1), one Ni(2), and one As(2) atom. In the third O site, O(7) is bonded in a trigonal planar geometry to one Ni(1), one Ni(2), and one As(2) atom. In the fourth O site, O(8) is bonded in a single-bond geometry to one Ni(1) atom. In the fifth O site, O(1) is bonded in a distorted trigonal planar geometry to one Ni(1), one Ni(2), and one As(1) atom. In the sixth O site, O(9) is bonded in a single-bond geometry to one Ni(2) atom. In the seventh O site, O(2) is bonded in a bent 120 degrees geometry to one Ni(1) and one As(1) atom. In the eighth O site, O(3) is bonded in a distorted trigonal planar geometry to one Ni(1), one Ni(2), and one As(1) atom. In the ninth O site, O(4) is bonded in a bent 120 degrees geometry to one As(1) and one As(2) atom.
Ni2As2O9 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(1), one O(2), one O(3), one O(6), one O(7), and one O(8) atom to form NiO6 octahedra that share corners with two equivalent As(2)O4 tetrahedra, corners with three equivalent As(1)O4 tetrahedra, and edges with two equivalent Ni(2)O6 octahedra. The Ni(1)-O(1) bond length is 2.11 Å. The Ni(1)-O(2) bond length is 2.02 Å. The Ni(1)-O(3) bond length is 2.10 Å. The Ni(1)-O(6) bond length is 2.09 Å. The Ni(1)-O(7) bond length is 2.10 Å. The Ni(1)-O(8) bond length is 1.87 Å. In the second Ni site, Ni(2) is bonded to one O(1), one O(3), one O(5), one O(6), one O(7), and one O(9) atom to form NiO6 octahedra that share corners with two equivalent As(1)O4 tetrahedra, corners with three equivalent As(2)O4 tetrahedra, and edges with two equivalent Ni(1)O6 octahedra. The Ni(2)-O(1) bond length is 2.13 Å. The Ni(2)-O(3) bond length is 2.05 Å. The Ni(2)-O(5) bond length is 2.02 Å. The Ni(2)-O(6) bond length is 2.09 Å. The Ni(2)-O(7) bond length is 2.09 Å. The Ni(2)-O(9) bond length is 1.86 Å. There are two inequivalent As sites. In the first As site, As(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form AsO4 tetrahedra that share corners with two equivalent Ni(2)O6 octahedra, corners with three equivalent Ni(1)O6 octahedra, and  a cornercorner with one As(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-62°. The As(1)-O(1) bond length is 1.72 Å. The As(1)-O(2) bond length is 1.69 Å. The As(1)-O(3) bond length is 1.73 Å. The As(1)-O(4) bond length is 1.79 Å. In the second As site, As(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form AsO4 tetrahedra that share corners with two equivalent Ni(1)O6 octahedra, corners with three equivalent Ni(2)O6 octahedra, and  a cornercorner with one As(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 47-62°. The As(2)-O(4) bond length is 1.80 Å. The As(2)-O(5) bond length is 1.67 Å. The As(2)-O(6) bond length is 1.72 Å. The As(2)-O(7) bond length is 1.72 Å. There are nine inequivalent O sites. In the first O site, O(5) is bonded in a bent 120 degrees geometry to one Ni(2) and one As(2) atom. In the second O site, O(6) is bonded in a distorted trigonal planar geometry to one Ni(1), one Ni(2), and one As(2) atom. In the third O site, O(7) is bonded in a trigonal planar geometry to one Ni(1), one Ni(2), and one As(2) atom. In the fourth O site, O(8) is bonded in a single-bond geometry to one Ni(1) atom. In the fifth O site, O(1) is bonded in a distorted trigonal planar geometry to one Ni(1), one Ni(2), and one As(1) atom. In the sixth O site, O(9) is bonded in a single-bond geometry to one Ni(2) atom. In the seventh O site, O(2) is bonded in a bent 120 degrees geometry to one Ni(1) and one As(1) atom. In the eighth O site, O(3) is bonded in a distorted trigonal planar geometry to one Ni(1), one Ni(2), and one As(1) atom. In the ninth O site, O(4) is bonded in a bent 120 degrees geometry to one As(1) and one As(2) atom.