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NiSe is Millerite structured and crystallizes in the trigonal R3m space group. The structure is three-dimensional. Ni2+ is bonded to five equivalent Se2- atoms to form a mixture of edge and corner-sharing NiSe5 trigonal bipyramids. There are three shorter (2.39 Å) and two longer (2.47 Å) Ni–Se bond lengths. Se2- is bonded in a 5-coordinate geometry to five equivalent Ni2+ atoms.
Ni3Se2 is Hazelwoodite structured and crystallizes in the trigonal R32 space group. The structure is three-dimensional. Ni+1.33+ is bonded to four equivalent Se2- atoms to form a mixture of distorted edge and corner-sharing NiSe4 tetrahedra. There are two shorter (2.37 Å) and two longer (2.39 Å) Ni–Se bond lengths. Se2- is bonded in a 6-coordinate geometry to six equivalent Ni+1.33+ atoms.
NiSe2 is pyrite-like structured and crystallizes in the cubic Pa-3 space group. The structure is three-dimensional. Ni4+ is bonded to six equivalent Se2- atoms to form corner-sharing NiSe6 octahedra. The corner-sharing octahedral tilt angles are 63°. All Ni–Se bond lengths are 2.46 Å. Se2- is bonded in a 3-coordinate geometry to three equivalent Ni4+ atoms.
Ni6Se5 crystallizes in the orthorhombic Pca2_1 space group. The structure is three-dimensional. there are twelve inequivalent Ni+1.67+ sites. In the first Ni+1.67+ site, Ni+1.67+ is bonded to five Se2- atoms to form distorted NiSe5 trigonal bipyramids that share corners with four NiSe4 tetrahedra, corners with ten NiSe5 trigonal bipyramids, edges with four NiSe5 trigonal bipyramids, and a faceface with one NiSe5 trigonal bipyramid. There are a spread of Ni–Se bond distances ranging from 2.39–2.49 Å. In the second Ni+1.67+ site, Ni+1.67+ is bonded to five Se2- atoms to form distorted NiSe5 trigonal bipyramids that share corners with four NiSe4 tetrahedra, corners with ten NiSe5 trigonal bipyramids, and edges with five NiSe5 trigonal bipyramids. There are a spread of Ni–Se bond distances ranging from 2.34–2.57 Å. In the third Ni+1.67+ site, Ni+1.67+ is bonded to five Se2- atoms to form distorted NiSe5 trigonal bipyramids that share corners with four NiSe4 tetrahedra, corners with eight NiSe5 trigonal bipyramids, and edges with six NiSe5 trigonal bipyramids. There are a spread of Ni–Se bond distances ranging from 2.34–2.50 Å. In the fourth Ni+1.67+ site, Ni+1.67+ is bonded to five Se2- atoms to form distorted NiSe5 trigonal bipyramids that share corners with two equivalent NiSe4 tetrahedra, corners with six NiSe5 trigonal bipyramids, edges with two NiSe4 tetrahedra, edges with four NiSe5 trigonal bipyramids, and a faceface with one NiSe5 trigonal bipyramid. There are a spread of Ni–Se bond distances ranging from 2.37–2.68 Å. In the fifth Ni+1.67+ site, Ni+1.67+ is bonded in a 5-coordinate geometry to five Se2- atoms. There are a spread of Ni–Se bond distances ranging from 2.38–2.80 Å. In the sixth Ni+1.67+ site, Ni+1.67+ is bonded to four Se2- atoms to form NiSe4 tetrahedra that share corners with two NiSe5 square pyramids, corners with two equivalent NiSe4 tetrahedra, corners with six NiSe5 trigonal bipyramids, an edgeedge with one NiSe4 tetrahedra, and edges with two NiSe5 trigonal bipyramids. There are a spread of Ni–Se bond distances ranging from 2.31–2.37 Å. In the seventh Ni+1.67+ site, Ni+1.67+ is bonded to five Se2- atoms to form NiSe5 square pyramids that share corners with two equivalent NiSe5 square pyramids, corners with three NiSe4 tetrahedra, corners with four equivalent NiSe5 trigonal bipyramids, edges with three equivalent NiSe5 square pyramids, and edges with two NiSe5 trigonal bipyramids. There are a spread of Ni–Se bond distances ranging from 2.32–2.49 Å. In the eighth Ni+1.67+ site, Ni+1.67+ is bonded to five Se2- atoms to form NiSe5 trigonal bipyramids that share corners with eight NiSe5 square pyramids, corners with two equivalent NiSe4 tetrahedra, corners with two equivalent NiSe5 trigonal bipyramids, edges with two NiSe5 square pyramids, and edges with two equivalent NiSe5 trigonal bipyramids. There are a spread of Ni–Se bond distances ranging from 2.40–2.53 Å. In the ninth Ni+1.67+ site, Ni+1.67+ is bonded to four Se2- atoms to form NiSe4 tetrahedra that share corners with three NiSe5 square pyramids, corners with nine NiSe5 trigonal bipyramids, and edges with two NiSe4 tetrahedra. There are three shorter (2.32 Å) and one longer (2.35 Å) Ni–Se bond lengths. In the tenth Ni+1.67+ site, Ni+1.67+ is bonded to five Se2- atoms to form NiSe5 trigonal bipyramids that share corners with seven NiSe4 tetrahedra, corners with four NiSe5 trigonal bipyramids, edges with two NiSe5 square pyramids, an edgeedge with one NiSe4 tetrahedra, and an edgeedge with one NiSe5 trigonal bipyramid. There are a spread of Ni–Se bond distances ranging from 2.38–2.46 Å. In the eleventh Ni+1.67+ site, Ni+1.67+ is bonded to five Se2- atoms to form distorted NiSe5 square pyramids that share corners with two equivalent NiSe5 square pyramids, corners with four NiSe4 tetrahedra, corners with four equivalent NiSe5 trigonal bipyramids, edges with three equivalent NiSe5 square pyramids, and edges with two NiSe5 trigonal bipyramids. There are a spread of Ni–Se bond distances ranging from 2.32–2.63 Å. In the twelfth Ni+1.67+ site, Ni+1.67+ is bonded to four Se2- atoms to form NiSe4 tetrahedra that share corners with two NiSe5 square pyramids, corners with two equivalent NiSe4 tetrahedra, corners with eight NiSe5 trigonal bipyramids, an edgeedge with one NiSe4 tetrahedra, and an edgeedge with one NiSe5 trigonal bipyramid. There are a spread of Ni–Se bond distances ranging from 2.28–2.35 Å. There are ten inequivalent Se2- sites. In the first Se2- site, Se2- is bonded in a 5-coordinate geometry to five Ni+1.67+ atoms. In the second Se2- site, Se2- is bonded in a distorted pentagonal planar geometry to five Ni+1.67+ atoms. In the third Se2- site, Se2- is bonded in a 6-coordinate geometry to six Ni+1.67+ atoms. In the fourth Se2- site, Se2- is bonded in a 6-coordinate geometry to six Ni+1.67+ atoms. In the fifth Se2- site, Se2- is bonded in a distorted hexagonal planar geometry to six Ni+1.67+ atoms. In the sixth Se2- site, Se2- is bonded in a 6-coordinate geometry to six Ni+1.67+ atoms. In the seventh Se2- site, Se2- is bonded in a 6-coordinate geometry to six Ni+1.67+ atoms. In the eighth Se2- site, Se2- is bonded in a 6-coordinate geometry to six Ni+1.67+ atoms. In the ninth Se2- site, Se2- is bonded in a 5-coordinate geometry to five Ni+1.67+ atoms. In the tenth Se2- site, Se2- is bonded in a distorted hexagonal planar geometry to six Ni+1.67+ atoms.
NiSe is lead oxide structured and crystallizes in the tetragonal P4/nmm space group. The structure is two-dimensional and consists of one NiSe sheet oriented in the (0, 0, 1) direction. Ni2+ is bonded to four equivalent Se2- atoms to form a mixture of edge and corner-sharing NiSe4 tetrahedra. All Ni–Se bond lengths are 2.36 Å. Se2- is bonded in a 4-coordinate geometry to four equivalent Ni2+ atoms.
Ni3Se4 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. there are two inequivalent Ni+2.67+ sites. In the first Ni+2.67+ site, Ni+2.67+ is bonded to six Se2- atoms to form a mixture of edge, face, and corner-sharing NiSe6 octahedra. The corner-sharing octahedra tilt angles range from 49–54°. There are two shorter (2.45 Å) and four longer (2.50 Å) Ni–Se bond lengths. In the second Ni+2.67+ site, Ni+2.67+ is bonded to six Se2- atoms to form a mixture of edge, face, and corner-sharing NiSe6 octahedra. The corner-sharing octahedra tilt angles range from 49–54°. There are a spread of Ni–Se bond distances ranging from 2.40–2.50 Å. There are two inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to four Ni+2.67+ atoms to form a mixture of distorted edge and corner-sharing SeNi4 trigonal pyramids. In the second Se2- site, Se2- is bonded in a 5-coordinate geometry to five Ni+2.67+ atoms.
NiSe is Molybdenum Carbide MAX Phase-like structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Ni2+ is bonded to six equivalent Se2- atoms to form a mixture of edge, corner, and face-sharing NiSe6 octahedra. The corner-sharing octahedral tilt angles are 50°. All Ni–Se bond lengths are 2.50 Å. Se2- is bonded in a 6-coordinate geometry to six equivalent Ni2+ atoms.
NiSe2 is Marcasite structured and crystallizes in the orthorhombic Pnnm space group. The structure is three-dimensional. Ni4+ is bonded to six equivalent Se2- atoms to form a mixture of edge and corner-sharing NiSe6 octahedra. The corner-sharing octahedral tilt angles are 59°. All Ni–Se bond lengths are 2.46 Å. Se2- is bonded in a 4-coordinate geometry to three equivalent Ni4+ atoms.
Ni3Se is Uranium Silicide-like structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. there are two inequivalent Ni sites. In the first Ni site, Ni is bonded to eight equivalent Ni and four equivalent Se atoms to form distorted NiNi8Se4 cuboctahedra that share corners with four equivalent SeNi12 cuboctahedra, corners with fourteen NiNi8Se4 cuboctahedra, edges with six equivalent SeNi12 cuboctahedra, edges with twelve equivalent NiNi8Se4 cuboctahedra, faces with four equivalent SeNi12 cuboctahedra, and faces with sixteen NiNi8Se4 cuboctahedra. There are a spread of Ni–Ni bond distances ranging from 2.54–2.62 Å. All Ni–Se bond lengths are 2.58 Å. In the second Ni site, Ni is bonded to eight Ni and four equivalent Se atoms to form distorted NiNi8Se4 cuboctahedra that share corners with four equivalent SeNi12 cuboctahedra, corners with fourteen NiNi8Se4 cuboctahedra, edges with six equivalent SeNi12 cuboctahedra, edges with twelve NiNi8Se4 cuboctahedra, faces with four equivalent SeNi12 cuboctahedra, and faces with sixteen NiNi8Se4 cuboctahedra. There are a spread of Ni–Ni bond distances ranging from 2.54–2.62 Å. All Ni–Se bond lengths are 2.58 Å. Se is bonded to twelve Ni atoms to form SeNi12 cuboctahedra that share corners with six equivalent SeNi12 cuboctahedra, corners with twelve NiNi8Se4 cuboctahedra, edges with eighteen NiNi8Se4 cuboctahedra, faces with eight equivalent SeNi12 cuboctahedra, and faces with twelve NiNi8Se4 cuboctahedra.
Ni3Se4 is Hausmannite-like structured and crystallizes in the orthorhombic Imma space group. The structure is three-dimensional. there are three inequivalent Ni+2.67+ sites. In the first Ni+2.67+ site, Ni+2.67+ is bonded to four Se2- atoms to form corner-sharing NiSe4 tetrahedra. The corner-sharing octahedral tilt angles are 58°. All Ni–Se bond lengths are 2.32 Å. In the second Ni+2.67+ site, Ni+2.67+ is bonded to six Se2- atoms to form NiSe6 octahedra that share corners with six equivalent NiSe4 tetrahedra and edges with six NiSe6 octahedra. All Ni–Se bond lengths are 2.41 Å. In the third Ni+2.67+ site, Ni+2.67+ is bonded to six Se2- atoms to form NiSe6 octahedra that share corners with six equivalent NiSe4 tetrahedra and edges with six NiSe6 octahedra. All Ni–Se bond lengths are 2.41 Å. There are two inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to four Ni+2.67+ atoms to form a mixture of distorted edge and corner-sharing SeNi4 trigonal pyramids. In the second Se2- site, Se2- is bonded to four Ni+2.67+ atoms to form a mixture of distorted edge and corner-sharing SeNi4 trigonal pyramids.