Mg3Cr crystallizes in the orthorhombic Pmm2 space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(1), two equivalent Mg(2), four equivalent Mg(3), and four equivalent Cr(1) atoms to form MgMg8Cr4 cuboctahedra that share corners with eight equivalent Cr(1)Mg10Cr2 cuboctahedra, corners with ten equivalent Mg(1)Mg8Cr4 cuboctahedra, edges with two equivalent Mg(1)Mg8Cr4 cuboctahedra, edges with four equivalent Mg(2)Mg8Cr4 cuboctahedra, edges with four equivalent Cr(1)Mg10Cr2 cuboctahedra, edges with eight equivalent Mg(3)Mg10Cr2 cuboctahedra, faces with four equivalent Mg(3)Mg10Cr2 cuboctahedra, faces with four equivalent Mg(1)Mg8Cr4 cuboctahedra, faces with four equivalent Cr(1)Mg10Cr2 cuboctahedra, and faces with eight equivalent Mg(2)Mg8Cr4 cuboctahedra. In the second Mg site, Mg(2) is bonded to two equivalent Mg(1), two equivalent Mg(2), four equivalent Mg(3), and four equivalent Cr(1) atoms to form distorted MgMg8Cr4 cuboctahedra that share corners with eight equivalent Mg(3)Mg10Cr2 cuboctahedra, corners with ten equivalent Mg(2)Mg8Cr4 cuboctahedra, edges with two equivalent Mg(2)Mg8Cr4 cuboctahedra, edges with four equivalent Mg(3)Mg10Cr2 cuboctahedra, edges with four equivalent Mg(1)Mg8Cr4 cuboctahedra, edges with eight equivalent Cr(1)Mg10Cr2 cuboctahedra, faces with four equivalent Mg(3)Mg10Cr2 cuboctahedra, faces with four equivalent Mg(2)Mg8Cr4 cuboctahedra, faces with four equivalent Cr(1)Mg10Cr2 cuboctahedra, and faces with eight equivalent Mg(1)Mg8Cr4 cuboctahedra. In the third Mg site, Mg(3) is bonded to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Cr(1) atoms to form distorted MgMg10Cr2 cuboctahedra that share corners with eight equivalent Mg(2)Mg8Cr4 cuboctahedra, corners with ten equivalent Mg(3)Mg10Cr2 cuboctahedra, edges with two equivalent Mg(3)Mg10Cr2 cuboctahedra, edges with four equivalent Mg(2)Mg8Cr4 cuboctahedra, edges with four equivalent Cr(1)Mg10Cr2 cuboctahedra, edges with eight equivalent Mg(1)Mg8Cr4 cuboctahedra, faces with four equivalent Mg(3)Mg10Cr2 cuboctahedra, faces with four equivalent Mg(1)Mg8Cr4 cuboctahedra, faces with four equivalent Mg(2)Mg8Cr4 cuboctahedra, and faces with eight equivalent Cr(1)Mg10Cr2 cuboctahedra. Cr(1) is bonded to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Cr(1) atoms to form distorted CrMg10Cr2 cuboctahedra that share corners with eight equivalent Mg(1)Mg8Cr4 cuboctahedra, corners with ten equivalent Cr(1)Mg10Cr2 cuboctahedra, edges with two equivalent Cr(1)Mg10Cr2 cuboctahedra, edges with four equivalent Mg(3)Mg10Cr2 cuboctahedra, edges with four equivalent Mg(1)Mg8Cr4 cuboctahedra, edges with eight equivalent Mg(2)Mg8Cr4 cuboctahedra, faces with four equivalent Mg(1)Mg8Cr4 cuboctahedra, faces with four equivalent Mg(2)Mg8Cr4 cuboctahedra, faces with four equivalent Cr(1)Mg10Cr2 cuboctahedra, and faces with eight equivalent Mg(3)Mg10Cr2 cuboctahedra.
Mg3Cr crystallizes in the orthorhombic Pmm2 space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(1), two equivalent Mg(2), four equivalent Mg(3), and four equivalent Cr(1) atoms to form MgMg8Cr4 cuboctahedra that share corners with eight equivalent Cr(1)Mg10Cr2 cuboctahedra, corners with ten equivalent Mg(1)Mg8Cr4 cuboctahedra, edges with two equivalent Mg(1)Mg8Cr4 cuboctahedra, edges with four equivalent Mg(2)Mg8Cr4 cuboctahedra, edges with four equivalent Cr(1)Mg10Cr2 cuboctahedra, edges with eight equivalent Mg(3)Mg10Cr2 cuboctahedra, faces with four equivalent Mg(3)Mg10Cr2 cuboctahedra, faces with four equivalent Mg(1)Mg8Cr4 cuboctahedra, faces with four equivalent Cr(1)Mg10Cr2 cuboctahedra, and faces with eight equivalent Mg(2)Mg8Cr4 cuboctahedra. Both Mg(1)-Mg(1) bond lengths are 3.10 Å. Both Mg(1)-Mg(2) bond lengths are 3.06 Å. All Mg(1)-Mg(3) bond lengths are 3.08 Å. There are two shorter (3.07 Å) and two longer (3.11 Å) Mg(1)-Cr(1) bond lengths. In the second Mg site, Mg(2) is bonded to two equivalent Mg(1), two equivalent Mg(2), four equivalent Mg(3), and four equivalent Cr(1) atoms to form distorted MgMg8Cr4 cuboctahedra that share corners with eight equivalent Mg(3)Mg10Cr2 cuboctahedra, corners with ten equivalent Mg(2)Mg8Cr4 cuboctahedra, edges with two equivalent Mg(2)Mg8Cr4 cuboctahedra, edges with four equivalent Mg(3)Mg10Cr2 cuboctahedra, edges with four equivalent Mg(1)Mg8Cr4 cuboctahedra, edges with eight equivalent Cr(1)Mg10Cr2 cuboctahedra, faces with four equivalent Mg(3)Mg10Cr2 cuboctahedra, faces with four equivalent Mg(2)Mg8Cr4 cuboctahedra, faces with four equivalent Cr(1)Mg10Cr2 cuboctahedra, and faces with eight equivalent Mg(1)Mg8Cr4 cuboctahedra. Both Mg(2)-Mg(2) bond lengths are 3.10 Å. There are two shorter (3.04 Å) and two longer (3.14 Å) Mg(2)-Mg(3) bond lengths. All Mg(2)-Cr(1) bond lengths are 3.06 Å. In the third Mg site, Mg(3) is bonded to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Cr(1) atoms to form distorted MgMg10Cr2 cuboctahedra that share corners with eight equivalent Mg(2)Mg8Cr4 cuboctahedra, corners with ten equivalent Mg(3)Mg10Cr2 cuboctahedra, edges with two equivalent Mg(3)Mg10Cr2 cuboctahedra, edges with four equivalent Mg(2)Mg8Cr4 cuboctahedra, edges with four equivalent Cr(1)Mg10Cr2 cuboctahedra, edges with eight equivalent Mg(1)Mg8Cr4 cuboctahedra, faces with four equivalent Mg(3)Mg10Cr2 cuboctahedra, faces with four equivalent Mg(1)Mg8Cr4 cuboctahedra, faces with four equivalent Mg(2)Mg8Cr4 cuboctahedra, and faces with eight equivalent Cr(1)Mg10Cr2 cuboctahedra. Both Mg(3)-Mg(3) bond lengths are 3.10 Å. Both Mg(3)-Cr(1) bond lengths are 3.04 Å. Cr(1) is bonded to two equivalent Mg(3), four equivalent Mg(1), four equivalent Mg(2), and two equivalent Cr(1) atoms to form distorted CrMg10Cr2 cuboctahedra that share corners with eight equivalent Mg(1)Mg8Cr4 cuboctahedra, corners with ten equivalent Cr(1)Mg10Cr2 cuboctahedra, edges with two equivalent Cr(1)Mg10Cr2 cuboctahedra, edges with four equivalent Mg(3)Mg10Cr2 cuboctahedra, edges with four equivalent Mg(1)Mg8Cr4 cuboctahedra, edges with eight equivalent Mg(2)Mg8Cr4 cuboctahedra, faces with four equivalent Mg(1)Mg8Cr4 cuboctahedra, faces with four equivalent Mg(2)Mg8Cr4 cuboctahedra, faces with four equivalent Cr(1)Mg10Cr2 cuboctahedra, and faces with eight equivalent Mg(3)Mg10Cr2 cuboctahedra. Both Cr(1)-Cr(1) bond lengths are 3.10 Å.