Fe3NiAs8 crystallizes in the monoclinic P2/m space group. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one As(1), one As(2), two equivalent As(3), and two equivalent As(4) atoms to form FeAs6 octahedra that share corners with four equivalent Fe(2)As6 octahedra, corners with four equivalent Ni(1)As6 octahedra, and edges with two equivalent Fe(1)As6 octahedra. The corner-sharing octahedral tilt angles range from 53-55°. In the second Fe site, Fe(2) is bonded to two equivalent As(3) and four equivalent As(1) atoms to form a mixture of corner and edge-sharing FeAs6 octahedra. The corner-sharing octahedral tilt angles range from 53-54°. Ni(1) is bonded to two equivalent As(4) and four equivalent As(2) atoms to form NiAs6 octahedra that share corners with eight equivalent Fe(1)As6 octahedra and edges with two equivalent Ni(1)As6 octahedra. The corner-sharing octahedral tilt angles range from 53-55°. There are four inequivalent As sites. In the first As site, As(1) is bonded in a 3-coordinate geometry to one Fe(1) and two equivalent Fe(2) atoms. In the second As site, As(2) is bonded in a 3-coordinate geometry to one Fe(1) and two equivalent Ni(1) atoms. In the third As site, As(3) is bonded in a 3-coordinate geometry to one Fe(2) and two equivalent Fe(1) atoms. In the fourth As site, As(4) is bonded in a 3-coordinate geometry to two equivalent Fe(1) and one Ni(1) atom.
Fe3NiAs8 crystallizes in the monoclinic P2/m space group. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one As(1), one As(2), two equivalent As(3), and two equivalent As(4) atoms to form FeAs6 octahedra that share corners with four equivalent Fe(2)As6 octahedra, corners with four equivalent Ni(1)As6 octahedra, and edges with two equivalent Fe(1)As6 octahedra. The corner-sharing octahedral tilt angles range from 53-55°. The Fe(1)-As(1) bond length is 2.37 Å. The Fe(1)-As(2) bond length is 2.31 Å. Both Fe(1)-As(3) bond lengths are 2.36 Å. Both Fe(1)-As(4) bond lengths are 2.35 Å. In the second Fe site, Fe(2) is bonded to two equivalent As(3) and four equivalent As(1) atoms to form a mixture of corner and edge-sharing FeAs6 octahedra. The corner-sharing octahedral tilt angles range from 53-54°. Both Fe(2)-As(3) bond lengths are 2.36 Å. All Fe(2)-As(1) bond lengths are 2.37 Å. Ni(1) is bonded to two equivalent As(4) and four equivalent As(2) atoms to form NiAs6 octahedra that share corners with eight equivalent Fe(1)As6 octahedra and edges with two equivalent Ni(1)As6 octahedra. The corner-sharing octahedral tilt angles range from 53-55°. Both Ni(1)-As(4) bond lengths are 2.41 Å. All Ni(1)-As(2) bond lengths are 2.39 Å. There are four inequivalent As sites. In the first As site, As(1) is bonded in a 3-coordinate geometry to one Fe(1) and two equivalent Fe(2) atoms. In the second As site, As(2) is bonded in a 3-coordinate geometry to one Fe(1) and two equivalent Ni(1) atoms. In the third As site, As(3) is bonded in a 3-coordinate geometry to one Fe(2) and two equivalent Fe(1) atoms. In the fourth As site, As(4) is bonded in a 3-coordinate geometry to two equivalent Fe(1) and one Ni(1) atom.