ErFe6Sn6 crystallizes in the orthorhombic Cmcm space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to four equivalent Fe(1), four equivalent Fe(3), four equivalent Fe(5), one Sn(2), one Sn(5), two equivalent Sn(1), two equivalent Sn(3), and two equivalent Sn(9) atoms to form distorted ErFe12Sn8 hexagonal bipyramids that share corners with four equivalent Er(1)Fe12Sn8 hexagonal bipyramids, faces with two equivalent Fe(4)Er2Fe4Sn6 cuboctahedra, faces with eight equivalent Fe(1)Er2Fe4Sn6 cuboctahedra, faces with two equivalent Er(1)Fe12Sn8 hexagonal bipyramids, and faces with two equivalent Er(2)Fe12Sn8 hexagonal bipyramids. In the second Er site, Er(2) is bonded to four equivalent Fe(2), four equivalent Fe(4), four equivalent Fe(5), one Sn(3), two equivalent Sn(10), two equivalent Sn(5), and three equivalent Sn(8) atoms to form distorted ErFe12Sn8 hexagonal bipyramids that share faces with six equivalent Fe(4)Er2Fe4Sn6 cuboctahedra, faces with eight equivalent Fe(2)Er2Fe4Sn6 cuboctahedra, faces with two equivalent Er(1)Fe12Sn8 hexagonal bipyramids, and faces with four equivalent Er(2)Fe12Sn8 hexagonal bipyramids. There are five inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to two equivalent Er(1), two equivalent Fe(1), two equivalent Fe(3), two equivalent Sn(1), two equivalent Sn(2), and two equivalent Sn(9) atoms to form distorted FeEr2Fe4Sn6 cuboctahedra that share corners with four equivalent Fe(1)Er2Fe4Sn6 cuboctahedra, edges with two equivalent Fe(1)Er2Fe4Sn6 cuboctahedra, faces with four equivalent Fe(1)Er2Fe4Sn6 cuboctahedra, and faces with four equivalent Er(1)Fe12Sn8 hexagonal bipyramids. In the second Fe site, Fe(2) is bonded to two equivalent Er(2), two equivalent Fe(2), two equivalent Fe(4), two equivalent Sn(10), two equivalent Sn(7), and two equivalent Sn(8) atoms to form distorted FeEr2Fe4Sn6 cuboctahedra that share corners with four equivalent Fe(2)Er2Fe4Sn6 cuboctahedra, corners with four equivalent Fe(4)Er2Fe4Sn6 cuboctahedra, edges with two equivalent Fe(4)Er2Fe4Sn6 cuboctahedra, edges with three equivalent Fe(2)Er2Fe4Sn6 cuboctahedra, faces with three equivalent Fe(2)Er2Fe4Sn6 cuboctahedra, faces with four equivalent Fe(4)Er2Fe4Sn6 cuboctahedra, and faces with four equivalent Er(2)Fe12Sn8 hexagonal bipyramids. In the third Fe site, Fe(3) is bonded in a 12-coordinate geometry to two equivalent Er(1), two equivalent Fe(1), two equivalent Fe(5), one Sn(1), one Sn(2), one Sn(3), one Sn(4), and two equivalent Sn(9) atoms. In the fourth Fe site, Fe(4) is bonded to two equivalent Er(2), two equivalent Fe(2), two equivalent Fe(5), one Sn(5), one Sn(6), one Sn(7), one Sn(8), and two equivalent Sn(10) atoms to form distorted FeEr2Fe4Sn6 cuboctahedra that share corners with four equivalent Fe(2)Er2Fe4Sn6 cuboctahedra, corners with four equivalent Fe(4)Er2Fe4Sn6 cuboctahedra, edges with two equivalent Fe(2)Er2Fe4Sn6 cuboctahedra, edges with three equivalent Fe(4)Er2Fe4Sn6 cuboctahedra,  a faceface with one Fe(4)Er2Fe4Sn6 cuboctahedra, faces with four equivalent Fe(2)Er2Fe4Sn6 cuboctahedra,  a faceface with one Er(1)Fe12Sn8 hexagonal bipyramid, and faces with three equivalent Er(2)Fe12Sn8 hexagonal bipyramids. In the fifth Fe site, Fe(5) is bonded in a 12-coordinate geometry to one Er(1), one Er(2), one Fe(3), one Fe(4), two equivalent Fe(5), one Sn(10), one Sn(3), one Sn(4), one Sn(5), one Sn(6), and one Sn(9) atom. There are ten inequivalent Sn sites. In the first Sn site, Sn(5) is bonded in a 9-coordinate geometry to one Er(1), two equivalent Er(2), two equivalent Fe(4), and four equivalent Fe(5) atoms. In the second Sn site, Sn(6) is bonded in a 6-coordinate geometry to two equivalent Fe(4) and four equivalent Fe(5) atoms. In the third Sn site, Sn(2) is bonded in a 7-coordinate geometry to one Er(1), two equivalent Fe(3), and four equivalent Fe(1) atoms. In the fourth Sn site, Sn(7) is bonded in a 6-coordinate geometry to two equivalent Fe(4) and four equivalent Fe(2) atoms. In the fifth Sn site, Sn(8) is bonded in a 9-coordinate geometry to three equivalent Er(2), two equivalent Fe(4), and four equivalent Fe(2) atoms. In the sixth Sn site, Sn(9) is bonded in a 8-coordinate geometry to one Er(1), two equivalent Fe(1), two equivalent Fe(3), two equivalent Fe(5), and one Sn(9) atom. In the seventh Sn site, Sn(10) is bonded in a 8-coordinate geometry to one Er(2), two equivalent Fe(2), two equivalent Fe(4), two equivalent Fe(5), and one Sn(10) atom. In the eighth Sn site, Sn(1) is bonded in a 8-coordinate geometry to two equivalent Er(1), two equivalent Fe(3), and four equivalent Fe(1) atoms. In the ninth Sn site, Sn(3) is bonded in a 9-coordinate geometry to one Er(2), two equivalent Er(1), two equivalent Fe(3), and four equivalent Fe(5) atoms. In the tenth Sn site, Sn(4) is bonded in a 6-coordinate geometry to two equivalent Fe(3) and four equivalent Fe(5) atoms.
ErFe6Sn6 crystallizes in the orthorhombic Cmcm space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to four equivalent Fe(1), four equivalent Fe(3), four equivalent Fe(5), one Sn(2), one Sn(5), two equivalent Sn(1), two equivalent Sn(3), and two equivalent Sn(9) atoms to form distorted ErFe12Sn8 hexagonal bipyramids that share corners with four equivalent Er(1)Fe12Sn8 hexagonal bipyramids, faces with two equivalent Fe(4)Er2Fe4Sn6 cuboctahedra, faces with eight equivalent Fe(1)Er2Fe4Sn6 cuboctahedra, faces with two equivalent Er(1)Fe12Sn8 hexagonal bipyramids, and faces with two equivalent Er(2)Fe12Sn8 hexagonal bipyramids. All Er(1)-Fe(1) bond lengths are 3.45 Å. All Er(1)-Fe(3) bond lengths are 3.50 Å. All Er(1)-Fe(5) bond lengths are 3.56 Å. The Er(1)-Sn(2) bond length is 3.10 Å. The Er(1)-Sn(5) bond length is 3.16 Å. Both Er(1)-Sn(1) bond lengths are 3.11 Å. Both Er(1)-Sn(3) bond lengths are 3.15 Å. Both Er(1)-Sn(9) bond lengths are 3.02 Å. In the second Er site, Er(2) is bonded to four equivalent Fe(2), four equivalent Fe(4), four equivalent Fe(5), one Sn(3), two equivalent Sn(10), two equivalent Sn(5), and three equivalent Sn(8) atoms to form distorted ErFe12Sn8 hexagonal bipyramids that share faces with six equivalent Fe(4)Er2Fe4Sn6 cuboctahedra, faces with eight equivalent Fe(2)Er2Fe4Sn6 cuboctahedra, faces with two equivalent Er(1)Fe12Sn8 hexagonal bipyramids, and faces with four equivalent Er(2)Fe12Sn8 hexagonal bipyramids. All Er(2)-Fe(2) bond lengths are 3.50 Å. All Er(2)-Fe(4) bond lengths are 3.49 Å. All Er(2)-Fe(5) bond lengths are 3.45 Å. The Er(2)-Sn(3) bond length is 3.14 Å. Both Er(2)-Sn(10) bond lengths are 3.02 Å. Both Er(2)-Sn(5) bond lengths are 3.13 Å. There are two shorter (3.13 Å) and one longer (3.15 Å) Er(2)-Sn(8) bond length. There are five inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to two equivalent Er(1), two equivalent Fe(1), two equivalent Fe(3), two equivalent Sn(1), two equivalent Sn(2), and two equivalent Sn(9) atoms to form distorted FeEr2Fe4Sn6 cuboctahedra that share corners with four equivalent Fe(1)Er2Fe4Sn6 cuboctahedra, edges with two equivalent Fe(1)Er2Fe4Sn6 cuboctahedra, faces with four equivalent Fe(1)Er2Fe4Sn6 cuboctahedra, and faces with four equivalent Er(1)Fe12Sn8 hexagonal bipyramids. Both Fe(1)-Fe(1) bond lengths are 2.71 Å. Both Fe(1)-Fe(3) bond lengths are 2.70 Å. Both Fe(1)-Sn(1) bond lengths are 2.71 Å. Both Fe(1)-Sn(2) bond lengths are 2.75 Å. Both Fe(1)-Sn(9) bond lengths are 2.83 Å. In the second Fe site, Fe(2) is bonded to two equivalent Er(2), two equivalent Fe(2), two equivalent Fe(4), two equivalent Sn(10), two equivalent Sn(7), and two equivalent Sn(8) atoms to form distorted FeEr2Fe4Sn6 cuboctahedra that share corners with four equivalent Fe(2)Er2Fe4Sn6 cuboctahedra, corners with four equivalent Fe(4)Er2Fe4Sn6 cuboctahedra, edges with two equivalent Fe(4)Er2Fe4Sn6 cuboctahedra, edges with three equivalent Fe(2)Er2Fe4Sn6 cuboctahedra, faces with three equivalent Fe(2)Er2Fe4Sn6 cuboctahedra, faces with four equivalent Fe(4)Er2Fe4Sn6 cuboctahedra, and faces with four equivalent Er(2)Fe12Sn8 hexagonal bipyramids. Both Fe(2)-Fe(2) bond lengths are 2.71 Å. Both Fe(2)-Fe(4) bond lengths are 2.70 Å. Both Fe(2)-Sn(10) bond lengths are 2.80 Å. Both Fe(2)-Sn(7) bond lengths are 2.76 Å. Both Fe(2)-Sn(8) bond lengths are 2.70 Å. In the third Fe site, Fe(3) is bonded in a 12-coordinate geometry to two equivalent Er(1), two equivalent Fe(1), two equivalent Fe(5), one Sn(1), one Sn(2), one Sn(3), one Sn(4), and two equivalent Sn(9) atoms. Both Fe(3)-Fe(5) bond lengths are 2.71 Å. The Fe(3)-Sn(1) bond length is 2.75 Å. The Fe(3)-Sn(2) bond length is 2.69 Å. The Fe(3)-Sn(3) bond length is 2.68 Å. The Fe(3)-Sn(4) bond length is 2.77 Å. Both Fe(3)-Sn(9) bond lengths are 2.82 Å. In the fourth Fe site, Fe(4) is bonded to two equivalent Er(2), two equivalent Fe(2), two equivalent Fe(5), one Sn(5), one Sn(6), one Sn(7), one Sn(8), and two equivalent Sn(10) atoms to form distorted FeEr2Fe4Sn6 cuboctahedra that share corners with four equivalent Fe(2)Er2Fe4Sn6 cuboctahedra, corners with four equivalent Fe(4)Er2Fe4Sn6 cuboctahedra, edges with two equivalent Fe(2)Er2Fe4Sn6 cuboctahedra, edges with three equivalent Fe(4)Er2Fe4Sn6 cuboctahedra,  a faceface with one Fe(4)Er2Fe4Sn6 cuboctahedra, faces with four equivalent Fe(2)Er2Fe4Sn6 cuboctahedra,  a faceface with one Er(1)Fe12Sn8 hexagonal bipyramid, and faces with three equivalent Er(2)Fe12Sn8 hexagonal bipyramids. Both Fe(4)-Fe(5) bond lengths are 2.68 Å. The Fe(4)-Sn(5) bond length is 2.71 Å. The Fe(4)-Sn(6) bond length is 2.75 Å. The Fe(4)-Sn(7) bond length is 2.77 Å. The Fe(4)-Sn(8) bond length is 2.69 Å. Both Fe(4)-Sn(10) bond lengths are 2.83 Å. In the fifth Fe site, Fe(5) is bonded in a 12-coordinate geometry to one Er(1), one Er(2), one Fe(3), one Fe(4), two equivalent Fe(5), one Sn(10), one Sn(3), one Sn(4), one Sn(5), one Sn(6), and one Sn(9) atom. There is one shorter (2.69 Å) and one longer (2.72 Å) Fe(5)-Fe(5) bond length. The Fe(5)-Sn(10) bond length is 2.82 Å. The Fe(5)-Sn(3) bond length is 2.71 Å. The Fe(5)-Sn(4) bond length is 2.75 Å. The Fe(5)-Sn(5) bond length is 2.69 Å. The Fe(5)-Sn(6) bond length is 2.76 Å. The Fe(5)-Sn(9) bond length is 2.81 Å. There are ten inequivalent Sn sites. In the first Sn site, Sn(5) is bonded in a 9-coordinate geometry to one Er(1), two equivalent Er(2), two equivalent Fe(4), and four equivalent Fe(5) atoms. In the second Sn site, Sn(6) is bonded in a 6-coordinate geometry to two equivalent Fe(4) and four equivalent Fe(5) atoms. In the third Sn site, Sn(2) is bonded in a 7-coordinate geometry to one Er(1), two equivalent Fe(3), and four equivalent Fe(1) atoms. In the fourth Sn site, Sn(7) is bonded in a 6-coordinate geometry to two equivalent Fe(4) and four equivalent Fe(2) atoms. In the fifth Sn site, Sn(8) is bonded in a 9-coordinate geometry to three equivalent Er(2), two equivalent Fe(4), and four equivalent Fe(2) atoms. In the sixth Sn site, Sn(9) is bonded in a 8-coordinate geometry to one Er(1), two equivalent Fe(1), two equivalent Fe(3), two equivalent Fe(5), and one Sn(9) atom. The Sn(9)-Sn(9) bond length is 2.92 Å. In the seventh Sn site, Sn(10) is bonded in a 8-coordinate geometry to one Er(2), two equivalent Fe(2), two equivalent Fe(4), two equivalent Fe(5), and one Sn(10) atom. The Sn(10)-Sn(10) bond length is 2.92 Å. In the eighth Sn site, Sn(1) is bonded in a 8-coordinate geometry to two equivalent Er(1), two equivalent Fe(3), and four equivalent Fe(1) atoms. In the ninth Sn site, Sn(3) is bonded in a 9-coordinate geometry to one Er(2), two equivalent Er(1), two equivalent Fe(3), and four equivalent Fe(5) atoms. In the tenth Sn site, Sn(4) is bonded in a 6-coordinate geometry to two equivalent Fe(3) and four equivalent Fe(5) atoms.