ErNdFe17 crystallizes in the trigonal R3m space group. Er(1) is bonded in a 19-coordinate geometry to one Fe(6), three equivalent Fe(1), three equivalent Fe(3), six equivalent Fe(2), and six equivalent Fe(4) atoms. Nd(1) is bonded in a 19-coordinate geometry to one Fe(5), three equivalent Fe(1), three equivalent Fe(2), six equivalent Fe(3), and six equivalent Fe(4) atoms. There are six inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one Er(1), one Nd(1), one Fe(5), one Fe(6), two equivalent Fe(2), two equivalent Fe(3), and four equivalent Fe(4) atoms to form FeNdErFe10 cuboctahedra that share corners with four equivalent Fe(1)NdErFe10 cuboctahedra, corners with five equivalent Fe(3)Nd2ErFe9 cuboctahedra, corners with five equivalent Fe(2)NdEr2Fe9 cuboctahedra, edges with three equivalent Fe(3)Nd2ErFe9 cuboctahedra, edges with three equivalent Fe(2)NdEr2Fe9 cuboctahedra, faces with three equivalent Fe(3)Nd2ErFe9 cuboctahedra, faces with three equivalent Fe(2)NdEr2Fe9 cuboctahedra, and faces with four equivalent Fe(1)NdErFe10 cuboctahedra. In the second Fe site, Fe(2) is bonded to two equivalent Er(1), one Nd(1), one Fe(6), two equivalent Fe(1), two equivalent Fe(3), and four equivalent Fe(4) atoms to form FeNdEr2Fe9 cuboctahedra that share corners with two equivalent Fe(2)NdEr2Fe9 cuboctahedra, corners with five equivalent Fe(1)NdErFe10 cuboctahedra, corners with eight equivalent Fe(3)Nd2ErFe9 cuboctahedra,  an edgeedge with one Fe(3)Nd2ErFe9 cuboctahedra, edges with three equivalent Fe(1)NdErFe10 cuboctahedra, edges with four equivalent Fe(2)NdEr2Fe9 cuboctahedra, faces with three equivalent Fe(3)Nd2ErFe9 cuboctahedra, faces with three equivalent Fe(1)NdErFe10 cuboctahedra, and faces with four equivalent Fe(2)NdEr2Fe9 cuboctahedra. In the third Fe site, Fe(3) is bonded to one Er(1), two equivalent Nd(1), one Fe(5), two equivalent Fe(1), two equivalent Fe(2), and four equivalent Fe(4) atoms to form FeNd2ErFe9 cuboctahedra that share corners with two equivalent Fe(3)Nd2ErFe9 cuboctahedra, corners with five equivalent Fe(1)NdErFe10 cuboctahedra, corners with eight equivalent Fe(2)NdEr2Fe9 cuboctahedra,  an edgeedge with one Fe(2)NdEr2Fe9 cuboctahedra, edges with three equivalent Fe(1)NdErFe10 cuboctahedra, edges with four equivalent Fe(3)Nd2ErFe9 cuboctahedra, faces with three equivalent Fe(2)NdEr2Fe9 cuboctahedra, faces with three equivalent Fe(1)NdErFe10 cuboctahedra, and faces with four equivalent Fe(3)Nd2ErFe9 cuboctahedra. In the fourth Fe site, Fe(4) is bonded in a 12-coordinate geometry to one Er(1), one Nd(1), one Fe(5), one Fe(6), two equivalent Fe(1), two equivalent Fe(2), two equivalent Fe(3), and two equivalent Fe(4) atoms. In the fifth Fe site, Fe(5) is bonded in a 14-coordinate geometry to one Nd(1), one Fe(6), three equivalent Fe(1), three equivalent Fe(3), and six equivalent Fe(4) atoms. In the sixth Fe site, Fe(6) is bonded in a 14-coordinate geometry to one Er(1), one Fe(5), three equivalent Fe(1), three equivalent Fe(2), and six equivalent Fe(4) atoms.
ErNdFe17 crystallizes in the trigonal R3m space group. Er(1) is bonded in a 19-coordinate geometry to one Fe(6), three equivalent Fe(1), three equivalent Fe(3), six equivalent Fe(2), and six equivalent Fe(4) atoms. The Er(1)-Fe(6) bond length is 3.03 Å. All Er(1)-Fe(1) bond lengths are 3.27 Å. All Er(1)-Fe(3) bond lengths are 3.07 Å. There are three shorter (3.19 Å) and three longer (3.24 Å) Er(1)-Fe(2) bond lengths. All Er(1)-Fe(4) bond lengths are 3.02 Å. Nd(1) is bonded in a 19-coordinate geometry to one Fe(5), three equivalent Fe(1), three equivalent Fe(2), six equivalent Fe(3), and six equivalent Fe(4) atoms. The Nd(1)-Fe(5) bond length is 3.06 Å. All Nd(1)-Fe(1) bond lengths are 3.30 Å. All Nd(1)-Fe(2) bond lengths are 3.08 Å. There are three shorter (3.20 Å) and three longer (3.22 Å) Nd(1)-Fe(3) bond lengths. All Nd(1)-Fe(4) bond lengths are 3.04 Å. There are six inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one Er(1), one Nd(1), one Fe(5), one Fe(6), two equivalent Fe(2), two equivalent Fe(3), and four equivalent Fe(4) atoms to form FeNdErFe10 cuboctahedra that share corners with four equivalent Fe(1)NdErFe10 cuboctahedra, corners with five equivalent Fe(3)Nd2ErFe9 cuboctahedra, corners with five equivalent Fe(2)NdEr2Fe9 cuboctahedra, edges with three equivalent Fe(3)Nd2ErFe9 cuboctahedra, edges with three equivalent Fe(2)NdEr2Fe9 cuboctahedra, faces with three equivalent Fe(3)Nd2ErFe9 cuboctahedra, faces with three equivalent Fe(2)NdEr2Fe9 cuboctahedra, and faces with four equivalent Fe(1)NdErFe10 cuboctahedra. The Fe(1)-Fe(5) bond length is 2.61 Å. The Fe(1)-Fe(6) bond length is 2.60 Å. Both Fe(1)-Fe(2) bond lengths are 2.45 Å. Both Fe(1)-Fe(3) bond lengths are 2.46 Å. There are two shorter (2.43 Å) and two longer (2.44 Å) Fe(1)-Fe(4) bond lengths. In the second Fe site, Fe(2) is bonded to two equivalent Er(1), one Nd(1), one Fe(6), two equivalent Fe(1), two equivalent Fe(3), and four equivalent Fe(4) atoms to form FeNdEr2Fe9 cuboctahedra that share corners with two equivalent Fe(2)NdEr2Fe9 cuboctahedra, corners with five equivalent Fe(1)NdErFe10 cuboctahedra, corners with eight equivalent Fe(3)Nd2ErFe9 cuboctahedra,  an edgeedge with one Fe(3)Nd2ErFe9 cuboctahedra, edges with three equivalent Fe(1)NdErFe10 cuboctahedra, edges with four equivalent Fe(2)NdEr2Fe9 cuboctahedra, faces with three equivalent Fe(3)Nd2ErFe9 cuboctahedra, faces with three equivalent Fe(1)NdErFe10 cuboctahedra, and faces with four equivalent Fe(2)NdEr2Fe9 cuboctahedra. The Fe(2)-Fe(6) bond length is 2.63 Å. Both Fe(2)-Fe(3) bond lengths are 2.48 Å. There are two shorter (2.53 Å) and two longer (2.64 Å) Fe(2)-Fe(4) bond lengths. In the third Fe site, Fe(3) is bonded to one Er(1), two equivalent Nd(1), one Fe(5), two equivalent Fe(1), two equivalent Fe(2), and four equivalent Fe(4) atoms to form FeNd2ErFe9 cuboctahedra that share corners with two equivalent Fe(3)Nd2ErFe9 cuboctahedra, corners with five equivalent Fe(1)NdErFe10 cuboctahedra, corners with eight equivalent Fe(2)NdEr2Fe9 cuboctahedra,  an edgeedge with one Fe(2)NdEr2Fe9 cuboctahedra, edges with three equivalent Fe(1)NdErFe10 cuboctahedra, edges with four equivalent Fe(3)Nd2ErFe9 cuboctahedra, faces with three equivalent Fe(2)NdEr2Fe9 cuboctahedra, faces with three equivalent Fe(1)NdErFe10 cuboctahedra, and faces with four equivalent Fe(3)Nd2ErFe9 cuboctahedra. The Fe(3)-Fe(5) bond length is 2.65 Å. There are two shorter (2.55 Å) and two longer (2.63 Å) Fe(3)-Fe(4) bond lengths. In the fourth Fe site, Fe(4) is bonded in a 12-coordinate geometry to one Er(1), one Nd(1), one Fe(5), one Fe(6), two equivalent Fe(1), two equivalent Fe(2), two equivalent Fe(3), and two equivalent Fe(4) atoms. The Fe(4)-Fe(5) bond length is 2.76 Å. The Fe(4)-Fe(6) bond length is 2.76 Å. There is one shorter (2.47 Å) and one longer (2.50 Å) Fe(4)-Fe(4) bond length. In the fifth Fe site, Fe(5) is bonded in a 14-coordinate geometry to one Nd(1), one Fe(6), three equivalent Fe(1), three equivalent Fe(3), and six equivalent Fe(4) atoms. The Fe(5)-Fe(6) bond length is 2.41 Å. In the sixth Fe site, Fe(6) is bonded in a 14-coordinate geometry to one Er(1), one Fe(5), three equivalent Fe(1), three equivalent Fe(2), and six equivalent Fe(4) atoms.