FeNiSe4 is pyrite-derived structured and crystallizes in the monoclinic P2_1/c space group. Fe(1) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form FeSe6 octahedra that share corners with four equivalent Fe(1)Se6 octahedra and corners with eight equivalent Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 61-64°. Ni(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form NiSe6 octahedra that share corners with four equivalent Ni(1)Se6 octahedra and corners with eight equivalent Fe(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 62-65°. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 3-coordinate geometry to one Fe(1) and two equivalent Ni(1) atoms. In the second Se site, Se(2) is bonded in a distorted trigonal planar geometry to two equivalent Fe(1) and one Ni(1) atom.
FeNiSe4 is pyrite-derived structured and crystallizes in the monoclinic P2_1/c space group. Fe(1) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form FeSe6 octahedra that share corners with four equivalent Fe(1)Se6 octahedra and corners with eight equivalent Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 61-64°. Both Fe(1)-Se(1) bond lengths are 2.37 Å. There are two shorter (2.40 Å) and two longer (2.41 Å) Fe(1)-Se(2) bond lengths. Ni(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form NiSe6 octahedra that share corners with four equivalent Ni(1)Se6 octahedra and corners with eight equivalent Fe(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 62-65°. Both Ni(1)-Se(2) bond lengths are 2.49 Å. All Ni(1)-Se(1) bond lengths are 2.46 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 3-coordinate geometry to one Fe(1) and two equivalent Ni(1) atoms. In the second Se site, Se(2) is bonded in a distorted trigonal planar geometry to two equivalent Fe(1) and one Ni(1) atom.