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HoC2 crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Ho is bonded in a distorted q4 geometry to ten equivalent C atoms. There are two shorter (2.44 Å) and eight longer (2.66 Å) Ho–C bond lengths. C is bonded in a 6-coordinate geometry to five equivalent Ho and one C atom. The C–C bond length is 1.30 Å.
Ho4C7 crystallizes in the monoclinic P2_1/c space group. The structure is three-dimensional. there are two inequivalent Ho3+ sites. In the first Ho3+ site, Ho3+ is bonded to seven C+1.71- atoms to form distorted HoC7 octahedra that share corners with two equivalent HoC7 octahedra, a cornercorner with one CHo4C trigonal bipyramid, and edges with five equivalent HoC7 octahedra. The corner-sharing octahedral tilt angles are 0°. There are a spread of Ho–C bond distances ranging from 2.48–2.99 Å. In the second Ho3+ site, Ho3+ is bonded in a 9-coordinate geometry to nine C+1.71- atoms. There are a spread of Ho–C bond distances ranging from 2.42–2.83 Å. There are four inequivalent C+1.71- sites. In the first C+1.71- site, C+1.71- is bonded to six Ho3+ atoms to form CHo6 octahedra that share corners with two equivalent CHo4C trigonal bipyramids, edges with two equivalent CHo6 octahedra, and edges with four equivalent CHo4C trigonal bipyramids. In the second C+1.71- site, C+1.71- is bonded to four Ho3+ and one C+1.71- atom to form distorted CHo4C trigonal bipyramids that share a cornercorner with one HoC7 octahedra, a cornercorner with one CHo6 octahedra, corners with six equivalent CHo4C trigonal bipyramids, and edges with two equivalent CHo6 octahedra. The corner-sharing octahedra tilt angles range from 33–61°. The C–C bond length is 1.33 Å. In the third C+1.71- site, C+1.71- is bonded in a 6-coordinate geometry to four Ho3+ and two C+1.71- atoms. The C–C bond length is 1.33 Å. In the fourth C+1.71- site, C+1.71- is bonded in a 5-coordinate geometry to five Ho3+ and one C+1.71- atom.
Ho4C5 crystallizes in the orthorhombic Pbam space group. The structure is three-dimensional. there are two inequivalent Ho3+ sites. In the first Ho3+ site, Ho3+ is bonded to five C+2.40- atoms to form HoC5 trigonal bipyramids that share corners with two equivalent CHo5C octahedra, corners with four equivalent HoC5 trigonal bipyramids, and edges with two equivalent HoC5 trigonal bipyramids. The corner-sharing octahedra tilt angles range from 36–48°. There are a spread of Ho–C bond distances ranging from 2.39–2.68 Å. In the second Ho3+ site, Ho3+ is bonded in a 7-coordinate geometry to nine C+2.40- atoms. There are a spread of Ho–C bond distances ranging from 2.44–2.86 Å. There are three inequivalent C+2.40- sites. In the first C+2.40- site, C+2.40- is bonded to six Ho3+ atoms to form CHo6 octahedra that share corners with four equivalent CHo5C octahedra and edges with six CHo6 octahedra. The corner-sharing octahedral tilt angles are 61°. In the second C+2.40- site, C+2.40- is bonded to five Ho3+ and one C+2.40- atom to form CHo5C octahedra that share corners with six CHo6 octahedra, corners with two equivalent HoC5 trigonal bipyramids, and edges with six CHo6 octahedra. The corner-sharing octahedra tilt angles range from 29–61°. The C–C bond length is 1.34 Å. In the third C+2.40- site, C+2.40- is bonded in a 5-coordinate geometry to six Ho3+ and one C+2.40- atom.
Ho2C is trigonal omega-like structured and crystallizes in the trigonal R-3m space group. The structure is two-dimensional and consists of three Ho2C sheets oriented in the (0, 0, 1) direction. Ho is bonded in a distorted T-shaped geometry to three equivalent C atoms. All Ho–C bond lengths are 2.46 Å. C is bonded to six equivalent Ho atoms to form edge-sharing CHo6 octahedra.
Ho2C3 is Plutonium carbide structured and crystallizes in the cubic I-43d space group. The structure is three-dimensional. Ho3+ is bonded in a 6-coordinate geometry to nine equivalent C2- atoms. There are a spread of Ho–C bond distances ranging from 2.50–2.80 Å. C2- is bonded to six equivalent Ho3+ and one C2- atom to form a mixture of distorted corner, edge, and face-sharing CHo6C trigonal bipyramids. The C–C bond length is 1.34 Å.
Ho3C4 crystallizes in the tetragonal P4/mnc space group. The structure is three-dimensional. there are four inequivalent Ho3+ sites. In the first Ho3+ site, Ho3+ is bonded to six C+2.25- atoms to form HoC6 octahedra that share corners with two equivalent HoC6 octahedra, corners with four equivalent HoC7 pentagonal bipyramids, and edges with four equivalent HoC7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 0°. There are two shorter (2.49 Å) and four longer (2.60 Å) Ho–C bond lengths. In the second Ho3+ site, Ho3+ is bonded to six C+2.25- atoms to form HoC6 octahedra that share a cornercorner with one HoC6 octahedra and edges with four equivalent HoC7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 0°. There are a spread of Ho–C bond distances ranging from 2.33–2.62 Å. In the third Ho3+ site, Ho3+ is bonded to seven C+2.25- atoms to form distorted HoC7 pentagonal bipyramids that share a cornercorner with one HoC6 octahedra, corners with two equivalent HoC7 pentagonal bipyramids, edges with three HoC6 octahedra, edges with two equivalent HoC7 pentagonal bipyramids, and faces with two equivalent HoC7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 1°. There are a spread of Ho–C bond distances ranging from 2.48–2.74 Å. In the fourth Ho3+ site, Ho3+ is bonded in a 7-coordinate geometry to seven C+2.25- atoms. There are a spread of Ho–C bond distances ranging from 2.34–2.83 Å. There are seven inequivalent C+2.25- sites. In the first C+2.25- site, C+2.25- is bonded to six Ho3+ atoms to form a mixture of edge and corner-sharing CHo6 octahedra. The corner-sharing octahedra tilt angles range from 0–85°. In the second C+2.25- site, C+2.25- is bonded to six Ho3+ atoms to form a mixture of edge and corner-sharing CHo6 octahedra. The corner-sharing octahedra tilt angles range from 0–1°. Both C–Ho bond lengths are 2.34 Å. In the third C+2.25- site, C+2.25- is bonded to six Ho3+ atoms to form a mixture of edge and corner-sharing CHo6 octahedra. The corner-sharing octahedra tilt angles range from 0–1°. Both C–Ho bond lengths are 2.34 Å. In the fourth C+2.25- site, C+2.25- is bonded to six Ho3+ atoms to form a mixture of edge and corner-sharing CHo6 octahedra. The corner-sharing octahedra tilt angles range from 0–1°. In the fifth C+2.25- site, C+2.25- is bonded to five Ho3+ and one C+2.25- atom to form a mixture of edge and corner-sharing CHo5C octahedra. The corner-sharing octahedra tilt angles range from 4–85°. The C–C bond length is 1.35 Å. In the sixth C+2.25- site, C+2.25- is bonded in a 2-coordinate geometry to four equivalent Ho3+ and two equivalent C+2.25- atoms. In the seventh C+2.25- site, C+2.25- is bonded in a 6-coordinate geometry to five Ho3+ and one C+2.25- atom. The C–C bond length is 1.31 Å.