Fe2NiSe4 crystallizes in the monoclinic C2/m space group. Fe(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form distorted FeSe6 octahedra that share corners with six equivalent Ni(1)Se6 octahedra, edges with six equivalent Fe(1)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 52-55°. Ni(1) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form NiSe6 octahedra that share corners with twelve equivalent Fe(1)Se6 octahedra, edges with two equivalent Ni(1)Se6 octahedra, and faces with two equivalent Fe(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 52-55°. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded in a 5-coordinate geometry to three equivalent Fe(1) and two equivalent Ni(1) atoms. In the second Se site, Se(1) is bonded in a distorted rectangular see-saw-like geometry to three equivalent Fe(1) and one Ni(1) atom.
Fe2NiSe4 crystallizes in the monoclinic C2/m space group. Fe(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form distorted FeSe6 octahedra that share corners with six equivalent Ni(1)Se6 octahedra, edges with six equivalent Fe(1)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 52-55°. There is one shorter (2.37 Å) and two longer (2.41 Å) Fe(1)-Se(1) bond lengths. There are two shorter (2.51 Å) and one longer (2.63 Å) Fe(1)-Se(2) bond length. Ni(1) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form NiSe6 octahedra that share corners with twelve equivalent Fe(1)Se6 octahedra, edges with two equivalent Ni(1)Se6 octahedra, and faces with two equivalent Fe(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 52-55°. Both Ni(1)-Se(1) bond lengths are 2.44 Å. All Ni(1)-Se(2) bond lengths are 2.52 Å. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded in a 5-coordinate geometry to three equivalent Fe(1) and two equivalent Ni(1) atoms. In the second Se site, Se(1) is bonded in a distorted rectangular see-saw-like geometry to three equivalent Fe(1) and one Ni(1) atom.