Er2Co7 crystallizes in the trigonal R-3m space group. There are six inequivalent Er sites. In the first Er site, Er(1) is bonded in a 18-coordinate geometry to three equivalent Co(1); three equivalent Co(4); six equivalent Co(2); and six Co(3,3) atoms. In the second Er site, Er(2) is bonded in a 12-coordinate geometry to three equivalent Co(5) and nine equivalent Co(2) atoms. In the third Er site, Er(1) is bonded in a 18-coordinate geometry to three equivalent Co(1); three equivalent Co(4); six equivalent Co(2); and six Co(3,3) atoms. In the fourth Er site, Er(1) is bonded in a 18-coordinate geometry to three equivalent Co(1); three equivalent Co(4); six equivalent Co(2); and six Co(3,3,3) atoms. In the fifth Er site, Er(1) is bonded in a 18-coordinate geometry to three equivalent Co(1); three equivalent Co(4); six equivalent Co(2); and six Co(3,3) atoms. In the sixth Er site, Er(1) is bonded in a 18-coordinate geometry to three equivalent Co(1); three equivalent Co(4); six equivalent Co(2); and six Co(3,3,3) atoms. There are eight inequivalent Co sites. In the first Co site, Co(3) is bonded to four Er(1,1); two equivalent Co(1); two equivalent Co(4); and four Co(3,3) atoms to form CoEr4Co8 cuboctahedra that share corners with four equivalent Co(3)Er4Co8 cuboctahedra; corners with twelve equivalent Co(2)Er5Co7 cuboctahedra; edges with two equivalent Co(3)Er4Co8 cuboctahedra; edges with eight equivalent Co(2)Er5Co7 cuboctahedra; faces with two equivalent Co(2)Er5Co7 cuboctahedra; and faces with eight Co(3,3)Er4Co8 cuboctahedra. In the second Co site, Co(3) is bonded to four Er(1,1); two equivalent Co(1); two equivalent Co(4); and four Co(3,3) atoms to form CoEr4Co8 cuboctahedra that share corners with four equivalent Co(3)Er4Co8 cuboctahedra; corners with twelve equivalent Co(2)Er5Co7 cuboctahedra; edges with two equivalent Co(3)Er4Co8 cuboctahedra; edges with eight equivalent Co(2)Er5Co7 cuboctahedra; faces with two equivalent Co(2)Er5Co7 cuboctahedra; and faces with eight Co(3,3)Er4Co8 cuboctahedra. In the third Co site, Co(1) is bonded in a 9-coordinate geometry to three equivalent Er(1); three equivalent Co(2); and three Co(3,3) atoms. In the fourth Co site, Co(2) is bonded to two equivalent Er(1), three equivalent Er(2), one Co(1), one Co(4), one Co(5), and four equivalent Co(2) atoms to form distorted CoEr5Co7 cuboctahedra that share corners with two equivalent Co(5)Er6Co6 cuboctahedra; corners with six Co(3,3)Er4Co8 cuboctahedra; corners with nine equivalent Co(2)Er5Co7 cuboctahedra; edges with four Co(3,3)Er4Co8 cuboctahedra; edges with four equivalent Co(2)Er5Co7 cuboctahedra;  a faceface with one Co(3)Er4Co8 cuboctahedra; faces with three equivalent Co(5)Er6Co6 cuboctahedra; and faces with ten equivalent Co(2)Er5Co7 cuboctahedra. In the fifth Co site, Co(3) is bonded to four equivalent Er(1), two equivalent Co(1), two equivalent Co(4), and four equivalent Co(3) atoms to form CoEr4Co8 cuboctahedra that share corners with four equivalent Co(3)Er4Co8 cuboctahedra, corners with twelve equivalent Co(2)Er5Co7 cuboctahedra, edges with two equivalent Co(3)Er4Co8 cuboctahedra, edges with eight equivalent Co(2)Er5Co7 cuboctahedra, faces with two equivalent Co(2)Er5Co7 cuboctahedra, and faces with eight equivalent Co(3)Er4Co8 cuboctahedra. In the sixth Co site, Co(4) is bonded in a 9-coordinate geometry to three equivalent Er(1); three equivalent Co(2); and three Co(3,3) atoms. In the seventh Co site, Co(5) is bonded to six equivalent Er(2) and six equivalent Co(2) atoms to form CoEr6Co6 cuboctahedra that share corners with twelve equivalent Co(2)Er5Co7 cuboctahedra, edges with six equivalent Co(5)Er6Co6 cuboctahedra, and faces with eighteen equivalent Co(2)Er5Co7 cuboctahedra. In the eighth Co site, Co(3) is bonded to four equivalent Er(1); two equivalent Co(1); two equivalent Co(4); and four Co(3,3) atoms to form CoEr4Co8 cuboctahedra that share corners with four equivalent Co(3)Er4Co8 cuboctahedra; corners with twelve equivalent Co(2)Er5Co7 cuboctahedra; edges with two equivalent Co(3)Er4Co8 cuboctahedra; edges with eight equivalent Co(2)Er5Co7 cuboctahedra; faces with two equivalent Co(2)Er5Co7 cuboctahedra; and faces with eight Co(3,3)Er4Co8 cuboctahedra.
Er2Co7 crystallizes in the trigonal R-3m space group. There are six inequivalent Er sites. In the first Er site, Er(1) is bonded in a 18-coordinate geometry to three equivalent Co(1); three equivalent Co(4); six equivalent Co(2); and six Co(3,3) atoms. All Er(1)-Co(1) bond lengths are 2.86 Å. All Er(1)-Co(4) bond lengths are 2.85 Å. All Er(1)-Co(2) bond lengths are 3.24 Å. All Er(1)-Co(3,3) bond lengths are 3.09 Å. In the second Er site, Er(2) is bonded in a 12-coordinate geometry to three equivalent Co(5) and nine equivalent Co(2) atoms. All Er(2)-Co(5) bond lengths are 2.93 Å. There are six shorter (2.83 Å) and three longer (3.08 Å) Er(2)-Co(2) bond lengths. In the third Er site, Er(1) is bonded in a 18-coordinate geometry to three equivalent Co(1); three equivalent Co(4); six equivalent Co(2); and six Co(3,3) atoms. All Er(1)-Co(1) bond lengths are 2.86 Å. All Er(1)-Co(4) bond lengths are 2.85 Å. All Er(1)-Co(2) bond lengths are 3.24 Å. All Er(1)-Co(3,3) bond lengths are 3.09 Å. In the fourth Er site, Er(1) is bonded in a 18-coordinate geometry to three equivalent Co(1); three equivalent Co(4); six equivalent Co(2); and six Co(3,3,3) atoms. All Er(1)-Co(1) bond lengths are 2.86 Å. All Er(1)-Co(4) bond lengths are 2.85 Å. All Er(1)-Co(2) bond lengths are 3.24 Å. All Er(1)-Co(3,3,3) bond lengths are 3.09 Å. In the fifth Er site, Er(1) is bonded in a 18-coordinate geometry to three equivalent Co(1); three equivalent Co(4); six equivalent Co(2); and six Co(3,3) atoms. All Er(1)-Co(1) bond lengths are 2.86 Å. All Er(1)-Co(4) bond lengths are 2.85 Å. All Er(1)-Co(2) bond lengths are 3.24 Å. All Er(1)-Co(3,3) bond lengths are 3.09 Å. In the sixth Er site, Er(1) is bonded in a 18-coordinate geometry to three equivalent Co(1); three equivalent Co(4); six equivalent Co(2); and six Co(3,3,3) atoms. All Er(1)-Co(1) bond lengths are 2.86 Å. All Er(1)-Co(4) bond lengths are 2.85 Å. All Er(1)-Co(2) bond lengths are 3.24 Å. All Er(1)-Co(3,3,3) bond lengths are 3.09 Å. There are eight inequivalent Co sites. In the first Co site, Co(3) is bonded to four Er(1,1); two equivalent Co(1); two equivalent Co(4); and four Co(3,3) atoms to form CoEr4Co8 cuboctahedra that share corners with four equivalent Co(3)Er4Co8 cuboctahedra; corners with twelve equivalent Co(2)Er5Co7 cuboctahedra; edges with two equivalent Co(3)Er4Co8 cuboctahedra; edges with eight equivalent Co(2)Er5Co7 cuboctahedra; faces with two equivalent Co(2)Er5Co7 cuboctahedra; and faces with eight Co(3,3)Er4Co8 cuboctahedra. Both Co(3)-Co(1) bond lengths are 2.46 Å. Both Co(3)-Co(4) bond lengths are 2.44 Å. All Co(3)-Co(3,3) bond lengths are 2.47 Å. In the second Co site, Co(3) is bonded to four Er(1,1); two equivalent Co(1); two equivalent Co(4); and four Co(3,3) atoms to form CoEr4Co8 cuboctahedra that share corners with four equivalent Co(3)Er4Co8 cuboctahedra; corners with twelve equivalent Co(2)Er5Co7 cuboctahedra; edges with two equivalent Co(3)Er4Co8 cuboctahedra; edges with eight equivalent Co(2)Er5Co7 cuboctahedra; faces with two equivalent Co(2)Er5Co7 cuboctahedra; and faces with eight Co(3,3)Er4Co8 cuboctahedra. Both Co(3)-Co(1) bond lengths are 2.46 Å. Both Co(3)-Co(4) bond lengths are 2.44 Å. Both Co(3)-Co(3) bond lengths are 2.47 Å. In the third Co site, Co(1) is bonded in a 9-coordinate geometry to three equivalent Er(1); three equivalent Co(2); and three Co(3,3) atoms. All Co(1)-Co(2) bond lengths are 2.42 Å. All Co(1)-Co(3,3) bond lengths are 2.46 Å. In the fourth Co site, Co(2) is bonded to two equivalent Er(1), three equivalent Er(2), one Co(1), one Co(4), one Co(5), and four equivalent Co(2) atoms to form distorted CoEr5Co7 cuboctahedra that share corners with two equivalent Co(5)Er6Co6 cuboctahedra; corners with six Co(3,3)Er4Co8 cuboctahedra; corners with nine equivalent Co(2)Er5Co7 cuboctahedra; edges with four Co(3,3)Er4Co8 cuboctahedra; edges with four equivalent Co(2)Er5Co7 cuboctahedra;  a faceface with one Co(3)Er4Co8 cuboctahedra; faces with three equivalent Co(5)Er6Co6 cuboctahedra; and faces with ten equivalent Co(2)Er5Co7 cuboctahedra. The Co(2)-Co(4) bond length is 2.43 Å. The Co(2)-Co(5) bond length is 2.51 Å. There are two shorter (2.46 Å) and two longer (2.48 Å) Co(2)-Co(2) bond lengths. In the fifth Co site, Co(3) is bonded to four equivalent Er(1), two equivalent Co(1), two equivalent Co(4), and four equivalent Co(3) atoms to form CoEr4Co8 cuboctahedra that share corners with four equivalent Co(3)Er4Co8 cuboctahedra, corners with twelve equivalent Co(2)Er5Co7 cuboctahedra, edges with two equivalent Co(3)Er4Co8 cuboctahedra, edges with eight equivalent Co(2)Er5Co7 cuboctahedra, faces with two equivalent Co(2)Er5Co7 cuboctahedra, and faces with eight equivalent Co(3)Er4Co8 cuboctahedra. Both Co(3)-Co(4) bond lengths are 2.44 Å. All Co(3)-Co(3) bond lengths are 2.47 Å. In the sixth Co site, Co(4) is bonded in a 9-coordinate geometry to three equivalent Er(1); three equivalent Co(2); and three Co(3,3) atoms. In the seventh Co site, Co(5) is bonded to six equivalent Er(2) and six equivalent Co(2) atoms to form CoEr6Co6 cuboctahedra that share corners with twelve equivalent Co(2)Er5Co7 cuboctahedra, edges with six equivalent Co(5)Er6Co6 cuboctahedra, and faces with eighteen equivalent Co(2)Er5Co7 cuboctahedra. In the eighth Co site, Co(3) is bonded to four equivalent Er(1); two equivalent Co(1); two equivalent Co(4); and four Co(3,3) atoms to form CoEr4Co8 cuboctahedra that share corners with four equivalent Co(3)Er4Co8 cuboctahedra; corners with twelve equivalent Co(2)Er5Co7 cuboctahedra; edges with two equivalent Co(3)Er4Co8 cuboctahedra; edges with eight equivalent Co(2)Er5Co7 cuboctahedra; faces with two equivalent Co(2)Er5Co7 cuboctahedra; and faces with eight Co(3,3)Er4Co8 cuboctahedra. Both Co(3)-Co(4) bond lengths are 2.44 Å. Both Co(3)-Co(3) bond lengths are 2.47 Å.