MgEu5 crystallizes in the hexagonal P-62m space group. Mg(1) is bonded to six equivalent Eu(1) and six equivalent Eu(2) atoms to form MgEu12 cuboctahedra that share corners with eighteen equivalent Eu(1)Eu9Mg3 cuboctahedra, edges with six equivalent Mg(1)Eu12 cuboctahedra, edges with twelve equivalent Eu(2)Eu10Mg2 cuboctahedra, faces with two equivalent Mg(1)Eu12 cuboctahedra, faces with six equivalent Eu(1)Eu9Mg3 cuboctahedra, and faces with twelve equivalent Eu(2)Eu10Mg2 cuboctahedra. There are two inequivalent Eu sites. In the first Eu site, Eu(1) is bonded to three equivalent Mg(1), three equivalent Eu(1), and six equivalent Eu(2) atoms to form EuEu9Mg3 cuboctahedra that share corners with nine equivalent Mg(1)Eu12 cuboctahedra, corners with nine equivalent Eu(1)Eu9Mg3 cuboctahedra, edges with six equivalent Eu(1)Eu9Mg3 cuboctahedra, edges with twelve equivalent Eu(2)Eu10Mg2 cuboctahedra, faces with three equivalent Mg(1)Eu12 cuboctahedra, faces with five equivalent Eu(1)Eu9Mg3 cuboctahedra, and faces with twelve equivalent Eu(2)Eu10Mg2 cuboctahedra. In the second Eu site, Eu(2) is bonded to two equivalent Mg(1), four equivalent Eu(1), and six equivalent Eu(2) atoms to form EuEu10Mg2 cuboctahedra that share corners with eighteen equivalent Eu(2)Eu10Mg2 cuboctahedra, edges with four equivalent Mg(1)Eu12 cuboctahedra, edges with six equivalent Eu(2)Eu10Mg2 cuboctahedra, edges with eight equivalent Eu(1)Eu9Mg3 cuboctahedra, faces with four equivalent Mg(1)Eu12 cuboctahedra, faces with eight equivalent Eu(2)Eu10Mg2 cuboctahedra, and faces with eight equivalent Eu(1)Eu9Mg3 cuboctahedra.
MgEu5 crystallizes in the hexagonal P-62m space group. Mg(1) is bonded to six equivalent Eu(1) and six equivalent Eu(2) atoms to form MgEu12 cuboctahedra that share corners with eighteen equivalent Eu(1)Eu9Mg3 cuboctahedra, edges with six equivalent Mg(1)Eu12 cuboctahedra, edges with twelve equivalent Eu(2)Eu10Mg2 cuboctahedra, faces with two equivalent Mg(1)Eu12 cuboctahedra, faces with six equivalent Eu(1)Eu9Mg3 cuboctahedra, and faces with twelve equivalent Eu(2)Eu10Mg2 cuboctahedra. All Mg(1)-Eu(1) bond lengths are 3.82 Å. All Mg(1)-Eu(2) bond lengths are 3.73 Å. There are two inequivalent Eu sites. In the first Eu site, Eu(1) is bonded to three equivalent Mg(1), three equivalent Eu(1), and six equivalent Eu(2) atoms to form EuEu9Mg3 cuboctahedra that share corners with nine equivalent Mg(1)Eu12 cuboctahedra, corners with nine equivalent Eu(1)Eu9Mg3 cuboctahedra, edges with six equivalent Eu(1)Eu9Mg3 cuboctahedra, edges with twelve equivalent Eu(2)Eu10Mg2 cuboctahedra, faces with three equivalent Mg(1)Eu12 cuboctahedra, faces with five equivalent Eu(1)Eu9Mg3 cuboctahedra, and faces with twelve equivalent Eu(2)Eu10Mg2 cuboctahedra. All Eu(1)-Eu(1) bond lengths are 3.82 Å. All Eu(1)-Eu(2) bond lengths are 3.80 Å. In the second Eu site, Eu(2) is bonded to two equivalent Mg(1), four equivalent Eu(1), and six equivalent Eu(2) atoms to form EuEu10Mg2 cuboctahedra that share corners with eighteen equivalent Eu(2)Eu10Mg2 cuboctahedra, edges with four equivalent Mg(1)Eu12 cuboctahedra, edges with six equivalent Eu(2)Eu10Mg2 cuboctahedra, edges with eight equivalent Eu(1)Eu9Mg3 cuboctahedra, faces with four equivalent Mg(1)Eu12 cuboctahedra, faces with eight equivalent Eu(2)Eu10Mg2 cuboctahedra, and faces with eight equivalent Eu(1)Eu9Mg3 cuboctahedra. There are two shorter (3.69 Å) and four longer (3.89 Å) Eu(2)-Eu(2) bond lengths.