Er4C7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 9-coordinate geometry to one C(4), two equivalent C(1), three equivalent C(2), and three equivalent C(3) atoms. In the second Er site, Er(2) is bonded to one C(2), one C(3), two equivalent C(4), and three equivalent C(1) atoms to form distorted ErC7 octahedra that share corners with two equivalent Er(2)C7 octahedra,  a cornercorner with one C(2)Er4C trigonal bipyramid, and edges with five equivalent Er(2)C7 octahedra. The corner-sharing octahedra are not tilted. There are four inequivalent C sites. In the first C site, C(1) is bonded in a 6-coordinate geometry to two equivalent Er(1), three equivalent Er(2), and one C(3) atom. In the second C site, C(2) is bonded to one Er(2), three equivalent Er(1), and one C(3) atom to form distorted CEr4C trigonal bipyramids that share  a cornercorner with one Er(2)C7 octahedra,  a cornercorner with one C(4)Er6 octahedra, corners with six equivalent C(2)Er4C trigonal bipyramids, and edges with two equivalent C(4)Er6 octahedra. The corner-sharing octahedral tilt angles range from 32-61°. In the third C site, C(3) is bonded in a 6-coordinate geometry to one Er(2), three equivalent Er(1), one C(1), and one C(2) atom. In the fourth C site, C(4) is bonded to two equivalent Er(1) and four equivalent Er(2) atoms to form CEr6 octahedra that share corners with two equivalent C(2)Er4C trigonal bipyramids, edges with two equivalent C(4)Er6 octahedra, and edges with four equivalent C(2)Er4C trigonal bipyramids.
Er4C7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 9-coordinate geometry to one C(4), two equivalent C(1), three equivalent C(2), and three equivalent C(3) atoms. The Er(1)-C(4) bond length is 2.41 Å. There is one shorter (2.55 Å) and one longer (2.79 Å) Er(1)-C(1) bond length. There are a spread of Er(1)-C(2) bond distances ranging from 2.40-2.56 Å. There are a spread of Er(1)-C(3) bond distances ranging from 2.45-2.79 Å. In the second Er site, Er(2) is bonded to one C(2), one C(3), two equivalent C(4), and three equivalent C(1) atoms to form distorted ErC7 octahedra that share corners with two equivalent Er(2)C7 octahedra,  a cornercorner with one C(2)Er4C trigonal bipyramid, and edges with five equivalent Er(2)C7 octahedra. The corner-sharing octahedra are not tilted. The Er(2)-C(2) bond length is 2.45 Å. The Er(2)-C(3) bond length is 2.96 Å. There is one shorter (2.49 Å) and one longer (2.58 Å) Er(2)-C(4) bond length. There are a spread of Er(2)-C(1) bond distances ranging from 2.46-2.65 Å. There are four inequivalent C sites. In the first C site, C(1) is bonded in a 6-coordinate geometry to two equivalent Er(1), three equivalent Er(2), and one C(3) atom. The C(1)-C(3) bond length is 1.33 Å. In the second C site, C(2) is bonded to one Er(2), three equivalent Er(1), and one C(3) atom to form distorted CEr4C trigonal bipyramids that share  a cornercorner with one Er(2)C7 octahedra,  a cornercorner with one C(4)Er6 octahedra, corners with six equivalent C(2)Er4C trigonal bipyramids, and edges with two equivalent C(4)Er6 octahedra. The corner-sharing octahedral tilt angles range from 32-61°. The C(2)-C(3) bond length is 1.32 Å. In the third C site, C(3) is bonded in a 6-coordinate geometry to one Er(2), three equivalent Er(1), one C(1), and one C(2) atom. In the fourth C site, C(4) is bonded to two equivalent Er(1) and four equivalent Er(2) atoms to form CEr6 octahedra that share corners with two equivalent C(2)Er4C trigonal bipyramids, edges with two equivalent C(4)Er6 octahedra, and edges with four equivalent C(2)Er4C trigonal bipyramids.