Mg6CeNb crystallizes in the orthorhombic Amm2 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) 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 second Mg site, Mg(2) is bonded in a 12-coordinate geometry to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(3), and two equivalent Ce(1) atoms. In the third Mg site, Mg(3) 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 distorted MgCe2Mg8Nb2 cuboctahedra that share corners with four equivalent Mg(4)Mg10Nb2 cuboctahedra, corners with ten equivalent Mg(3)Ce2Mg8Nb2 cuboctahedra, edges with two equivalent Mg(3)Ce2Mg8Nb2 cuboctahedra, edges with two equivalent Mg(4)Mg10Nb2 cuboctahedra, faces with two equivalent Mg(4)Mg10Nb2 cuboctahedra, and faces with four equivalent Mg(3)Ce2Mg8Nb2 cuboctahedra. In the fourth Mg site, Mg(4) is bonded to two equivalent Mg(2), four equivalent Mg(1), four equivalent Mg(3), and two equivalent Nb(1) atoms to form distorted MgMg10Nb2 cuboctahedra that share corners with six equivalent Mg(4)Mg10Nb2 cuboctahedra, corners with eight equivalent Mg(3)Ce2Mg8Nb2 cuboctahedra, edges with four equivalent Mg(3)Ce2Mg8Nb2 cuboctahedra, faces with two equivalent Mg(4)Mg10Nb2 cuboctahedra, and faces with four equivalent Mg(3)Ce2Mg8Nb2 cuboctahedra. Ce(1) is bonded in a 2-coordinate geometry to two equivalent Mg(2), four equivalent Mg(1), four equivalent Mg(3), and two equivalent Nb(1) atoms. Nb(1) is bonded in a 2-coordinate geometry to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(3), and two equivalent Ce(1) atoms.
Mg6CeNb crystallizes in the orthorhombic Amm2 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) 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 (3.12 Å) and one longer (3.32 Å) Mg(1)-Mg(1) bond length. There is one shorter (3.18 Å) and one longer (3.47 Å) Mg(1)-Mg(2) bond length. Both Mg(1)-Mg(3) bond lengths are 2.97 Å. Both Mg(1)-Mg(4) bond lengths are 3.18 Å. There is one shorter (3.08 Å) and one longer (3.57 Å) Mg(1)-Ce(1) bond length. Both Mg(1)-Nb(1) bond lengths are 3.24 Å. In the second Mg site, Mg(2) is bonded in a 12-coordinate geometry to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(3), and two equivalent Ce(1) atoms. Both Mg(2)-Mg(4) bond lengths are 3.08 Å. All Mg(2)-Mg(3) bond lengths are 3.10 Å. Both Mg(2)-Ce(1) bond lengths are 3.25 Å. In the third Mg site, Mg(3) 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 distorted MgCe2Mg8Nb2 cuboctahedra that share corners with four equivalent Mg(4)Mg10Nb2 cuboctahedra, corners with ten equivalent Mg(3)Ce2Mg8Nb2 cuboctahedra, edges with two equivalent Mg(3)Ce2Mg8Nb2 cuboctahedra, edges with two equivalent Mg(4)Mg10Nb2 cuboctahedra, faces with two equivalent Mg(4)Mg10Nb2 cuboctahedra, and faces with four equivalent Mg(3)Ce2Mg8Nb2 cuboctahedra. There is one shorter (3.15 Å) and one longer (3.30 Å) Mg(3)-Mg(3) bond length. There is one shorter (3.30 Å) and one longer (3.35 Å) Mg(3)-Mg(4) bond length. Both Mg(3)-Ce(1) bond lengths are 3.30 Å. There is one shorter (3.15 Å) and one longer (3.50 Å) Mg(3)-Nb(1) bond length. In the fourth Mg site, Mg(4) is bonded to two equivalent Mg(2), four equivalent Mg(1), four equivalent Mg(3), and two equivalent Nb(1) atoms to form distorted MgMg10Nb2 cuboctahedra that share corners with six equivalent Mg(4)Mg10Nb2 cuboctahedra, corners with eight equivalent Mg(3)Ce2Mg8Nb2 cuboctahedra, edges with four equivalent Mg(3)Ce2Mg8Nb2 cuboctahedra, faces with two equivalent Mg(4)Mg10Nb2 cuboctahedra, and faces with four equivalent Mg(3)Ce2Mg8Nb2 cuboctahedra. Both Mg(4)-Nb(1) bond lengths are 3.23 Å. Ce(1) is bonded in a 2-coordinate geometry to two equivalent Mg(2), four equivalent Mg(1), four equivalent Mg(3), and two equivalent Nb(1) atoms. Both Ce(1)-Nb(1) bond lengths are 2.73 Å. Nb(1) is bonded in a 2-coordinate geometry to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(3), and two equivalent Ce(1) atoms.