MgEu3 crystallizes in the orthorhombic Pmm2 space group. Mg(1) is bonded in a distorted q6 geometry to two equivalent Eu(1), four equivalent Eu(2), and four equivalent Eu(3) atoms. There are three inequivalent Eu sites. In the first Eu site, Eu(1) is bonded to two equivalent Mg(1), two equivalent Eu(1), four equivalent Eu(2), and four equivalent Eu(3) atoms to form EuEu10Mg2 cuboctahedra that share corners with eight equivalent Eu(2)Eu8Mg4 cuboctahedra, corners with ten equivalent Eu(1)Eu10Mg2 cuboctahedra, edges with two equivalent Eu(1)Eu10Mg2 cuboctahedra, edges with four equivalent Eu(2)Eu8Mg4 cuboctahedra, edges with eight equivalent Eu(3)Eu8Mg4 cuboctahedra, faces with four equivalent Eu(1)Eu10Mg2 cuboctahedra, faces with four equivalent Eu(2)Eu8Mg4 cuboctahedra, and faces with four equivalent Eu(3)Eu8Mg4 cuboctahedra. In the second Eu site, Eu(2) is bonded to four equivalent Mg(1), two equivalent Eu(2), two equivalent Eu(3), and four equivalent Eu(1) atoms to form distorted EuEu8Mg4 cuboctahedra that share corners with eight equivalent Eu(1)Eu10Mg2 cuboctahedra, corners with ten equivalent Eu(2)Eu8Mg4 cuboctahedra, edges with two equivalent Eu(2)Eu8Mg4 cuboctahedra, edges with four equivalent Eu(1)Eu10Mg2 cuboctahedra, edges with four equivalent Eu(3)Eu8Mg4 cuboctahedra, faces with four equivalent Eu(1)Eu10Mg2 cuboctahedra, faces with four equivalent Eu(2)Eu8Mg4 cuboctahedra, and faces with eight equivalent Eu(3)Eu8Mg4 cuboctahedra. In the third Eu site, Eu(3) is bonded to four equivalent Mg(1), two equivalent Eu(2), two equivalent Eu(3), and four equivalent Eu(1) atoms to form distorted EuEu8Mg4 cuboctahedra that share corners with ten equivalent Eu(3)Eu8Mg4 cuboctahedra, edges with two equivalent Eu(3)Eu8Mg4 cuboctahedra, edges with four equivalent Eu(2)Eu8Mg4 cuboctahedra, edges with eight equivalent Eu(1)Eu10Mg2 cuboctahedra, faces with four equivalent Eu(1)Eu10Mg2 cuboctahedra, faces with four equivalent Eu(3)Eu8Mg4 cuboctahedra, and faces with eight equivalent Eu(2)Eu8Mg4 cuboctahedra.
MgEu3 crystallizes in the orthorhombic Pmm2 space group. Mg(1) is bonded in a distorted q6 geometry to two equivalent Eu(1), four equivalent Eu(2), and four equivalent Eu(3) atoms. Both Mg(1)-Eu(1) bond lengths are 3.77 Å. All Mg(1)-Eu(2) bond lengths are 3.70 Å. There are two shorter (3.67 Å) and two longer (3.70 Å) Mg(1)-Eu(3) bond lengths. There are three inequivalent Eu sites. In the first Eu site, Eu(1) is bonded to two equivalent Mg(1), two equivalent Eu(1), four equivalent Eu(2), and four equivalent Eu(3) atoms to form EuEu10Mg2 cuboctahedra that share corners with eight equivalent Eu(2)Eu8Mg4 cuboctahedra, corners with ten equivalent Eu(1)Eu10Mg2 cuboctahedra, edges with two equivalent Eu(1)Eu10Mg2 cuboctahedra, edges with four equivalent Eu(2)Eu8Mg4 cuboctahedra, edges with eight equivalent Eu(3)Eu8Mg4 cuboctahedra, faces with four equivalent Eu(1)Eu10Mg2 cuboctahedra, faces with four equivalent Eu(2)Eu8Mg4 cuboctahedra, and faces with four equivalent Eu(3)Eu8Mg4 cuboctahedra. Both Eu(1)-Eu(1) bond lengths are 3.97 Å. There are two shorter (3.62 Å) and two longer (3.75 Å) Eu(1)-Eu(2) bond lengths. All Eu(1)-Eu(3) bond lengths are 3.72 Å. In the second Eu site, Eu(2) is bonded to four equivalent Mg(1), two equivalent Eu(2), two equivalent Eu(3), and four equivalent Eu(1) atoms to form distorted EuEu8Mg4 cuboctahedra that share corners with eight equivalent Eu(1)Eu10Mg2 cuboctahedra, corners with ten equivalent Eu(2)Eu8Mg4 cuboctahedra, edges with two equivalent Eu(2)Eu8Mg4 cuboctahedra, edges with four equivalent Eu(1)Eu10Mg2 cuboctahedra, edges with four equivalent Eu(3)Eu8Mg4 cuboctahedra, faces with four equivalent Eu(1)Eu10Mg2 cuboctahedra, faces with four equivalent Eu(2)Eu8Mg4 cuboctahedra, and faces with eight equivalent Eu(3)Eu8Mg4 cuboctahedra. Both Eu(2)-Eu(2) bond lengths are 3.97 Å. Both Eu(2)-Eu(3) bond lengths are 3.81 Å. In the third Eu site, Eu(3) is bonded to four equivalent Mg(1), two equivalent Eu(2), two equivalent Eu(3), and four equivalent Eu(1) atoms to form distorted EuEu8Mg4 cuboctahedra that share corners with ten equivalent Eu(3)Eu8Mg4 cuboctahedra, edges with two equivalent Eu(3)Eu8Mg4 cuboctahedra, edges with four equivalent Eu(2)Eu8Mg4 cuboctahedra, edges with eight equivalent Eu(1)Eu10Mg2 cuboctahedra, faces with four equivalent Eu(1)Eu10Mg2 cuboctahedra, faces with four equivalent Eu(3)Eu8Mg4 cuboctahedra, and faces with eight equivalent Eu(2)Eu8Mg4 cuboctahedra. Both Eu(3)-Eu(3) bond lengths are 3.97 Å.