NiSi2Se4 crystallizes in the monoclinic C2/m space group. 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 Si(1)Se6 octahedra, edges with two equivalent Ni(1)Se6 octahedra, and faces with two equivalent Si(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 43-55°. Si(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form SiSe6 octahedra that share corners with six equivalent Ni(1)Se6 octahedra, edges with six equivalent Si(1)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 43-55°. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded to two equivalent Ni(1) and three equivalent Si(1) atoms to form a mixture of distorted edge and corner-sharing SeSi3Ni2 trigonal bipyramids. In the second Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Ni(1) and three equivalent Si(1) atoms.
NiSi2Se4 crystallizes in the monoclinic C2/m space group. 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 Si(1)Se6 octahedra, edges with two equivalent Ni(1)Se6 octahedra, and faces with two equivalent Si(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 43-55°. Both Ni(1)-Se(1) bond lengths are 2.91 Å. All Ni(1)-Se(2) bond lengths are 2.41 Å. Si(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form SiSe6 octahedra that share corners with six equivalent Ni(1)Se6 octahedra, edges with six equivalent Si(1)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 43-55°. There are two shorter (2.46 Å) and one longer (2.51 Å) Si(1)-Se(1) bond length. All Si(1)-Se(2) bond lengths are 2.72 Å. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded to two equivalent Ni(1) and three equivalent Si(1) atoms to form a mixture of distorted edge and corner-sharing SeSi3Ni2 trigonal bipyramids. In the second Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Ni(1) and three equivalent Si(1) atoms.