Mg6CeNb crystallizes in the orthorhombic Amm2 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Ce(1), and two equivalent Nb(1) atoms to form MgCe2Mg8Nb2 cuboctahedra that share corners with four equivalent Ce(1)Mg10Nb2 cuboctahedra, corners with four equivalent Nb(1)Ce2Mg10 cuboctahedra, corners with ten equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, edges with two equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, edges with two equivalent Ce(1)Mg10Nb2 cuboctahedra, edges with two equivalent Nb(1)Ce2Mg10 cuboctahedra, edges with four equivalent Mg(3)Ce2Mg10 cuboctahedra, faces with two equivalent Mg(3)Ce2Mg10 cuboctahedra, faces with two equivalent Ce(1)Mg10Nb2 cuboctahedra, faces with two equivalent Nb(1)Ce2Mg10 cuboctahedra, and faces with four equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra. In the second Mg site, Mg(2) is bonded in a 12-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Ce(1), and two equivalent Nb(1) atoms. In the third Mg site, Mg(3) is bonded to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Ce(1) atoms to form distorted MgCe2Mg10 cuboctahedra that share corners with six equivalent Mg(3)Ce2Mg10 cuboctahedra, edges with four equivalent Ce(1)Mg10Nb2 cuboctahedra, edges with eight equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, faces with two equivalent Mg(3)Ce2Mg10 cuboctahedra, faces with two equivalent Ce(1)Mg10Nb2 cuboctahedra, faces with four equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, and faces with six equivalent Nb(1)Ce2Mg10 cuboctahedra. In the fourth Mg site, Mg(4) is bonded in a 10-coordinate geometry to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Nb(1) atoms. Ce(1) is bonded to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Nb(1) atoms to form distorted CeMg10Nb2 cuboctahedra that share corners with four equivalent Nb(1)Ce2Mg10 cuboctahedra, corners with six equivalent Ce(1)Mg10Nb2 cuboctahedra, corners with eight equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, edges with two equivalent Nb(1)Ce2Mg10 cuboctahedra, edges with four equivalent Mg(3)Ce2Mg10 cuboctahedra, edges with four equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, faces with two equivalent Mg(3)Ce2Mg10 cuboctahedra, faces with two equivalent Ce(1)Mg10Nb2 cuboctahedra, faces with two equivalent Nb(1)Ce2Mg10 cuboctahedra, and faces with four equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra. Nb(1) is bonded to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Ce(1) atoms to form distorted NbCe2Mg10 cuboctahedra that share corners with four equivalent Ce(1)Mg10Nb2 cuboctahedra, corners with six equivalent Nb(1)Ce2Mg10 cuboctahedra, corners with eight equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, edges with two equivalent Ce(1)Mg10Nb2 cuboctahedra, edges with four equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, faces with two equivalent Ce(1)Mg10Nb2 cuboctahedra, faces with two equivalent Nb(1)Ce2Mg10 cuboctahedra, faces with four equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, and faces with six equivalent Mg(3)Ce2Mg10 cuboctahedra.
Mg6CeNb crystallizes in the orthorhombic Amm2 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Ce(1), and two equivalent Nb(1) atoms to form MgCe2Mg8Nb2 cuboctahedra that share corners with four equivalent Ce(1)Mg10Nb2 cuboctahedra, corners with four equivalent Nb(1)Ce2Mg10 cuboctahedra, corners with ten equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, edges with two equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, edges with two equivalent Ce(1)Mg10Nb2 cuboctahedra, edges with two equivalent Nb(1)Ce2Mg10 cuboctahedra, edges with four equivalent Mg(3)Ce2Mg10 cuboctahedra, faces with two equivalent Mg(3)Ce2Mg10 cuboctahedra, faces with two equivalent Ce(1)Mg10Nb2 cuboctahedra, faces with two equivalent Nb(1)Ce2Mg10 cuboctahedra, and faces with four equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra. There is one shorter (2.92 Å) and one longer (2.95 Å) Mg(1)-Mg(1) bond length. Both Mg(1)-Mg(2) bond lengths are 3.18 Å. Both Mg(1)-Mg(3) bond lengths are 3.15 Å. Both Mg(1)-Mg(4) bond lengths are 3.26 Å. There is one shorter (3.27 Å) and one longer (3.31 Å) Mg(1)-Ce(1) bond length. There is one shorter (3.22 Å) and one longer (3.35 Å) Mg(1)-Nb(1) bond length. In the second Mg site, Mg(2) is bonded in a 12-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Ce(1), and two equivalent Nb(1) atoms. There is one shorter (2.84 Å) and one longer (3.03 Å) Mg(2)-Mg(2) bond length. There is one shorter (3.20 Å) and one longer (3.37 Å) Mg(2)-Mg(3) bond length. There is one shorter (3.06 Å) and one longer (3.52 Å) Mg(2)-Mg(4) bond length. Both Mg(2)-Ce(1) bond lengths are 3.20 Å. Both Mg(2)-Nb(1) bond lengths are 3.13 Å. In the third Mg site, Mg(3) is bonded to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Ce(1) atoms to form distorted MgCe2Mg10 cuboctahedra that share corners with six equivalent Mg(3)Ce2Mg10 cuboctahedra, edges with four equivalent Ce(1)Mg10Nb2 cuboctahedra, edges with eight equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, faces with two equivalent Mg(3)Ce2Mg10 cuboctahedra, faces with two equivalent Ce(1)Mg10Nb2 cuboctahedra, faces with four equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, and faces with six equivalent Nb(1)Ce2Mg10 cuboctahedra. Both Mg(3)-Mg(4) bond lengths are 2.95 Å. Both Mg(3)-Ce(1) bond lengths are 3.21 Å. In the fourth Mg site, Mg(4) is bonded in a 10-coordinate geometry to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Nb(1) atoms. Both Mg(4)-Nb(1) bond lengths are 3.09 Å. Ce(1) is bonded to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Nb(1) atoms to form distorted CeMg10Nb2 cuboctahedra that share corners with four equivalent Nb(1)Ce2Mg10 cuboctahedra, corners with six equivalent Ce(1)Mg10Nb2 cuboctahedra, corners with eight equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, edges with two equivalent Nb(1)Ce2Mg10 cuboctahedra, edges with four equivalent Mg(3)Ce2Mg10 cuboctahedra, edges with four equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, faces with two equivalent Mg(3)Ce2Mg10 cuboctahedra, faces with two equivalent Ce(1)Mg10Nb2 cuboctahedra, faces with two equivalent Nb(1)Ce2Mg10 cuboctahedra, and faces with four equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra. Both Ce(1)-Nb(1) bond lengths are 2.94 Å. Nb(1) is bonded to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Ce(1) atoms to form distorted NbCe2Mg10 cuboctahedra that share corners with four equivalent Ce(1)Mg10Nb2 cuboctahedra, corners with six equivalent Nb(1)Ce2Mg10 cuboctahedra, corners with eight equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, edges with two equivalent Ce(1)Mg10Nb2 cuboctahedra, edges with four equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, faces with two equivalent Ce(1)Mg10Nb2 cuboctahedra, faces with two equivalent Nb(1)Ce2Mg10 cuboctahedra, faces with four equivalent Mg(1)Ce2Mg8Nb2 cuboctahedra, and faces with six equivalent Mg(3)Ce2Mg10 cuboctahedra.