Fe2CoSe4 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 Co(1)Se6 octahedra, edges with six equivalent Fe(1)Se6 octahedra, and  a faceface with one Co(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 52-55°. Co(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form CoSe6 octahedra that share corners with twelve equivalent Fe(1)Se6 octahedra, edges with two equivalent Co(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 4-coordinate geometry to three equivalent Fe(1) and one Co(1) atom. In the second Se site, Se(1) is bonded in a 5-coordinate geometry to three equivalent Fe(1) and two equivalent Co(1) atoms.
Fe2CoSe4 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 Co(1)Se6 octahedra, edges with six equivalent Fe(1)Se6 octahedra, and  a faceface with one Co(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 52-55°. There are two shorter (2.55 Å) and one longer (2.59 Å) Fe(1)-Se(1) bond length. There is one shorter (2.36 Å) and two longer (2.40 Å) Fe(1)-Se(2) bond lengths. Co(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form CoSe6 octahedra that share corners with twelve equivalent Fe(1)Se6 octahedra, edges with two equivalent Co(1)Se6 octahedra, and faces with two equivalent Fe(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 52-55°. Both Co(1)-Se(2) bond lengths are 2.41 Å. All Co(1)-Se(1) bond lengths are 2.44 Å. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded in a 4-coordinate geometry to three equivalent Fe(1) and one Co(1) atom. In the second Se site, Se(1) is bonded in a 5-coordinate geometry to three equivalent Fe(1) and two equivalent Co(1) atoms.