Sr2Mg1 crystallizes in the trigonal P-3m1 space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 3-coordinate geometry to three equivalent Sr(2) and three equivalent Mg(1) atoms. In the second Sr site, Sr(2) is bonded to three equivalent Sr(1) and nine equivalent Sr(2) atoms to form SrSr12 cuboctahedra that share corners with six equivalent Sr(2)Sr12 cuboctahedra, corners with six equivalent Mg(1)Sr3Mg3 pentagonal pyramids, edges with twelve equivalent Sr(2)Sr12 cuboctahedra, faces with twelve equivalent Sr(2)Sr12 cuboctahedra, and  a faceface with one Mg(1)Sr3Mg3 pentagonal pyramid. Mg(1) is bonded to three equivalent Sr(1) and three equivalent Mg(1) atoms to form distorted MgSr3Mg3 pentagonal pyramids that share corners with six equivalent Sr(2)Sr12 cuboctahedra, edges with six equivalent Mg(1)Sr3Mg3 pentagonal pyramids, and  a faceface with one Sr(2)Sr12 cuboctahedra.
Sr2Mg1 crystallizes in the trigonal P-3m1 space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 3-coordinate geometry to three equivalent Sr(2) and three equivalent Mg(1) atoms. All Sr(1)-Sr(2) bond lengths are 4.27 Å. All Sr(1)-Mg(1) bond lengths are 3.61 Å. In the second Sr site, Sr(2) is bonded to three equivalent Sr(1) and nine equivalent Sr(2) atoms to form SrSr12 cuboctahedra that share corners with six equivalent Sr(2)Sr12 cuboctahedra, corners with six equivalent Mg(1)Sr3Mg3 pentagonal pyramids, edges with twelve equivalent Sr(2)Sr12 cuboctahedra, faces with twelve equivalent Sr(2)Sr12 cuboctahedra, and  a faceface with one Mg(1)Sr3Mg3 pentagonal pyramid. There are three shorter (4.21 Å) and six longer (4.23 Å) Sr(2)-Sr(2) bond lengths. Mg(1) is bonded to three equivalent Sr(1) and three equivalent Mg(1) atoms to form distorted MgSr3Mg3 pentagonal pyramids that share corners with six equivalent Sr(2)Sr12 cuboctahedra, edges with six equivalent Mg(1)Sr3Mg3 pentagonal pyramids, and  a faceface with one Sr(2)Sr12 cuboctahedra. All Mg(1)-Mg(1) bond lengths are 3.05 Å.