Mg6NbCu 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 Nb(1), and two equivalent Cu(1) atoms to form distorted MgMg8Nb2Cu2 cuboctahedra that share corners with four equivalent Nb(1)Mg10Cu2 cuboctahedra, corners with four equivalent Cu(1)Mg10Nb2 cuboctahedra, corners with ten equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, edges with two equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, edges with two equivalent Nb(1)Mg10Cu2 cuboctahedra, edges with two equivalent Cu(1)Mg10Nb2 cuboctahedra, edges with four equivalent Mg(4)Mg10Nb2 cuboctahedra, faces with two equivalent Mg(4)Mg10Nb2 cuboctahedra, faces with two equivalent Nb(1)Mg10Cu2 cuboctahedra, faces with two equivalent Cu(1)Mg10Nb2 cuboctahedra, and faces with four equivalent Mg(1)Mg8Nb2Cu2 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 Nb(1), and two equivalent Cu(1) atoms. In the third Mg site, Mg(3) is bonded in a distorted water-like geometry to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Cu(1) atoms. In the fourth Mg site, Mg(4) is bonded to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Nb(1) atoms to form distorted MgMg10Nb2 cuboctahedra that share corners with six equivalent Mg(4)Mg10Nb2 cuboctahedra, edges with four equivalent Nb(1)Mg10Cu2 cuboctahedra, edges with eight equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, faces with two equivalent Mg(4)Mg10Nb2 cuboctahedra, faces with two equivalent Nb(1)Mg10Cu2 cuboctahedra, faces with four equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, and faces with six equivalent Cu(1)Mg10Nb2 cuboctahedra. Nb(1) is bonded to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Cu(1) atoms to form NbMg10Cu2 cuboctahedra that share corners with four equivalent Cu(1)Mg10Nb2 cuboctahedra, corners with six equivalent Nb(1)Mg10Cu2 cuboctahedra, corners with eight equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, edges with two equivalent Cu(1)Mg10Nb2 cuboctahedra, edges with four equivalent Mg(4)Mg10Nb2 cuboctahedra, edges with four equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, faces with two equivalent Mg(4)Mg10Nb2 cuboctahedra, faces with two equivalent Nb(1)Mg10Cu2 cuboctahedra, faces with two equivalent Cu(1)Mg10Nb2 cuboctahedra, and faces with four equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra. Cu(1) is bonded to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Nb(1) atoms to form CuMg10Nb2 cuboctahedra that share corners with four equivalent Nb(1)Mg10Cu2 cuboctahedra, corners with six equivalent Cu(1)Mg10Nb2 cuboctahedra, corners with eight equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, edges with two equivalent Nb(1)Mg10Cu2 cuboctahedra, edges with four equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, faces with two equivalent Nb(1)Mg10Cu2 cuboctahedra, faces with two equivalent Cu(1)Mg10Nb2 cuboctahedra, faces with four equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, and faces with six equivalent Mg(4)Mg10Nb2 cuboctahedra.
Mg6NbCu 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 Nb(1), and two equivalent Cu(1) atoms to form distorted MgMg8Nb2Cu2 cuboctahedra that share corners with four equivalent Nb(1)Mg10Cu2 cuboctahedra, corners with four equivalent Cu(1)Mg10Nb2 cuboctahedra, corners with ten equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, edges with two equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, edges with two equivalent Nb(1)Mg10Cu2 cuboctahedra, edges with two equivalent Cu(1)Mg10Nb2 cuboctahedra, edges with four equivalent Mg(4)Mg10Nb2 cuboctahedra, faces with two equivalent Mg(4)Mg10Nb2 cuboctahedra, faces with two equivalent Nb(1)Mg10Cu2 cuboctahedra, faces with two equivalent Cu(1)Mg10Nb2 cuboctahedra, and faces with four equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra. There is one shorter (2.98 Å) and one longer (2.99 Å) Mg(1)-Mg(1) bond length. Both Mg(1)-Mg(2) bond lengths are 2.97 Å. Both Mg(1)-Mg(3) bond lengths are 3.08 Å. Both Mg(1)-Mg(4) bond lengths are 3.03 Å. There is one shorter (3.10 Å) and one longer (3.14 Å) Mg(1)-Nb(1) bond length. There is one shorter (3.01 Å) and one longer (3.22 Å) Mg(1)-Cu(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 Nb(1), and two equivalent Cu(1) atoms. There is one shorter (2.94 Å) and one longer (3.03 Å) Mg(2)-Mg(2) bond length. There is one shorter (2.96 Å) and one longer (3.28 Å) Mg(2)-Mg(3) bond length. There is one shorter (3.05 Å) and one longer (3.19 Å) Mg(2)-Mg(4) bond length. Both Mg(2)-Nb(1) bond lengths are 3.00 Å. Both Mg(2)-Cu(1) bond lengths are 2.98 Å. In the third Mg site, Mg(3) is bonded in a distorted water-like geometry to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Cu(1) atoms. Both Mg(3)-Mg(4) bond lengths are 2.99 Å. Both Mg(3)-Cu(1) bond lengths are 2.92 Å. In the fourth Mg site, Mg(4) is bonded to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Nb(1) atoms to form distorted MgMg10Nb2 cuboctahedra that share corners with six equivalent Mg(4)Mg10Nb2 cuboctahedra, edges with four equivalent Nb(1)Mg10Cu2 cuboctahedra, edges with eight equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, faces with two equivalent Mg(4)Mg10Nb2 cuboctahedra, faces with two equivalent Nb(1)Mg10Cu2 cuboctahedra, faces with four equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, and faces with six equivalent Cu(1)Mg10Nb2 cuboctahedra. Both Mg(4)-Nb(1) bond lengths are 2.92 Å. Nb(1) is bonded to two equivalent Mg(4), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Cu(1) atoms to form NbMg10Cu2 cuboctahedra that share corners with four equivalent Cu(1)Mg10Nb2 cuboctahedra, corners with six equivalent Nb(1)Mg10Cu2 cuboctahedra, corners with eight equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, edges with two equivalent Cu(1)Mg10Nb2 cuboctahedra, edges with four equivalent Mg(4)Mg10Nb2 cuboctahedra, edges with four equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, faces with two equivalent Mg(4)Mg10Nb2 cuboctahedra, faces with two equivalent Nb(1)Mg10Cu2 cuboctahedra, faces with two equivalent Cu(1)Mg10Nb2 cuboctahedra, and faces with four equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra. Both Nb(1)-Cu(1) bond lengths are 2.99 Å. Cu(1) is bonded to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Nb(1) atoms to form CuMg10Nb2 cuboctahedra that share corners with four equivalent Nb(1)Mg10Cu2 cuboctahedra, corners with six equivalent Cu(1)Mg10Nb2 cuboctahedra, corners with eight equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, edges with two equivalent Nb(1)Mg10Cu2 cuboctahedra, edges with four equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, faces with two equivalent Nb(1)Mg10Cu2 cuboctahedra, faces with two equivalent Cu(1)Mg10Nb2 cuboctahedra, faces with four equivalent Mg(1)Mg8Nb2Cu2 cuboctahedra, and faces with six equivalent Mg(4)Mg10Nb2 cuboctahedra.