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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.
Fe(NiSe2)2 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. Fe3+ is bonded to six Se2- atoms to form FeSe6 octahedra that share corners with twelve equivalent NiSe6 octahedra, edges with two equivalent FeSe6 octahedra, and faces with two equivalent NiSe6 octahedra. The corner-sharing octahedra tilt angles range from 51–54°. There are two shorter (2.43 Å) and four longer (2.48 Å) Fe–Se bond lengths. Ni+2.50+ is bonded to six Se2- atoms to form NiSe6 octahedra that share corners with six equivalent FeSe6 octahedra, edges with six equivalent NiSe6 octahedra, and a faceface with one FeSe6 octahedra. The corner-sharing octahedra tilt angles range from 51–54°. There are a spread of Ni–Se bond distances ranging from 2.39–2.52 Å. There are two inequivalent Se2- sites. In the first Se2- site, Se2- is bonded in a distorted rectangular see-saw-like geometry to one Fe3+ and three equivalent Ni+2.50+ atoms. In the second Se2- site, Se2- is bonded in a 5-coordinate geometry to two equivalent Fe3+ and three equivalent Ni+2.50+ 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.
CoNi2Se4 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. Co4+ is bonded to six Se2- atoms to form CoSe6 octahedra that share corners with twelve equivalent NiSe6 octahedra, edges with two equivalent CoSe6 octahedra, and faces with two equivalent NiSe6 octahedra. The corner-sharing octahedra tilt angles range from 48–55°. There are two shorter (2.43 Å) and four longer (2.45 Å) Co–Se bond lengths. Ni2+ is bonded to six Se2- atoms to form NiSe6 octahedra that share corners with six equivalent CoSe6 octahedra, edges with six equivalent NiSe6 octahedra, and a faceface with one CoSe6 octahedra. The corner-sharing octahedra tilt angles range from 48–55°. There are a spread of Ni–Se bond distances ranging from 2.39–2.55 Å. There are two inequivalent Se2- sites. In the first Se2- site, Se2- is bonded in a 5-coordinate geometry to two equivalent Co4+ and three equivalent Ni2+ atoms. In the second Se2- site, Se2- is bonded to one Co4+ and three equivalent Ni2+ atoms to form a mixture of distorted corner and edge-sharing SeCoNi3 trigonal pyramids.
CoNi2Se4 crystallizes in the monoclinic Cm space group. The structure is three-dimensional. Co4+ is bonded to six Se2- atoms to form CoSe6 octahedra that share corners with six equivalent NiSe6 octahedra, edges with two equivalent CoSe6 octahedra, edges with four equivalent NiSe6 octahedra, and a faceface with one NiSe6 octahedra. The corner-sharing octahedra tilt angles range from 49–54°. There are a spread of Co–Se bond distances ranging from 2.36–2.48 Å. There are two inequivalent Ni2+ sites. In the first Ni2+ site, Ni2+ is bonded to six Se2- atoms to form NiSe6 octahedra that share corners with six equivalent CoSe6 octahedra, corners with six equivalent NiSe6 octahedra, edges with two equivalent NiSe6 octahedra, a faceface with one CoSe6 octahedra, and a faceface with one NiSe6 octahedra. The corner-sharing octahedra tilt angles range from 49–54°. There are a spread of Ni–Se bond distances ranging from 2.41–2.51 Å. In the second Ni2+ site, Ni2+ is bonded to six Se2- atoms to form NiSe6 octahedra that share corners with six equivalent NiSe6 octahedra, edges with two equivalent NiSe6 octahedra, edges with four equivalent CoSe6 octahedra, and a faceface with one NiSe6 octahedra. The corner-sharing octahedra tilt angles range from 50–53°. There are a spread of Ni–Se bond distances ranging from 2.38–2.49 Å. There are four inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to one Co4+ and three Ni2+ atoms to form distorted SeCoNi3 trigonal pyramids that share corners with three SeCoNi3 trigonal pyramids and edges with two equivalent SeCo2Ni2 trigonal pyramids. In the second Se2- site, Se2- is bonded to two equivalent Co4+ and two Ni2+ atoms to form a mixture of distorted corner and edge-sharing SeCo2Ni2 trigonal pyramids. In the third Se2- site, Se2- is bonded in a 5-coordinate geometry to one Co4+ and four Ni2+ atoms. In the fourth Se2- site, Se2- is bonded in a 5-coordinate geometry to two equivalent Co4+ and three Ni2+ atoms.