Mg2Si crystallizes in the monoclinic C2/m space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(2), two equivalent Mg(3), two equivalent Si(1), two equivalent Si(2), and two equivalent Si(3) atoms to form distorted MgMg4Si6 cuboctahedra that share corners with two equivalent Mg(3)Mg8Si4 cuboctahedra, corners with four equivalent Mg(1)Mg4Si6 cuboctahedra, edges with two equivalent Mg(1)Mg4Si6 cuboctahedra, edges with four equivalent Mg(3)Mg8Si4 cuboctahedra, faces with two equivalent Mg(1)Mg4Si6 cuboctahedra, and faces with two equivalent Mg(3)Mg8Si4 cuboctahedra. In the second Mg site, Mg(2) is bonded in a 12-coordinate geometry to one Mg(1), three equivalent Mg(2), three equivalent Mg(3), one Si(2), one Si(3), and three equivalent Si(1) atoms. In the third Mg site, Mg(3) is bonded to two equivalent Mg(1), six equivalent Mg(2), one Si(2), and three equivalent Si(1) atoms to form distorted MgMg8Si4 cuboctahedra that share corners with two equivalent Mg(1)Mg4Si6 cuboctahedra, corners with nine equivalent Mg(3)Mg8Si4 cuboctahedra, edges with four equivalent Mg(1)Mg4Si6 cuboctahedra, faces with two equivalent Mg(1)Mg4Si6 cuboctahedra, and faces with three equivalent Mg(3)Mg8Si4 cuboctahedra. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded in a 11-coordinate geometry to two equivalent Mg(1), three equivalent Mg(3), and six equivalent Mg(2) atoms. In the second Si site, Si(2) is bonded in a distorted q6 geometry to two equivalent Mg(3), four equivalent Mg(1), and four equivalent Mg(2) atoms. In the third Si site, Si(3) is bonded in a distorted body-centered cubic geometry to four equivalent Mg(1) and four equivalent Mg(2) atoms.
Mg2Si crystallizes in the monoclinic C2/m space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Mg(2), two equivalent Mg(3), two equivalent Si(1), two equivalent Si(2), and two equivalent Si(3) atoms to form distorted MgMg4Si6 cuboctahedra that share corners with two equivalent Mg(3)Mg8Si4 cuboctahedra, corners with four equivalent Mg(1)Mg4Si6 cuboctahedra, edges with two equivalent Mg(1)Mg4Si6 cuboctahedra, edges with four equivalent Mg(3)Mg8Si4 cuboctahedra, faces with two equivalent Mg(1)Mg4Si6 cuboctahedra, and faces with two equivalent Mg(3)Mg8Si4 cuboctahedra. Both Mg(1)-Mg(2) bond lengths are 2.95 Å. Both Mg(1)-Mg(3) bond lengths are 3.02 Å. Both Mg(1)-Si(1) bond lengths are 2.96 Å. Both Mg(1)-Si(2) bond lengths are 2.95 Å. Both Mg(1)-Si(3) bond lengths are 2.95 Å. In the second Mg site, Mg(2) is bonded in a 12-coordinate geometry to one Mg(1), three equivalent Mg(2), three equivalent Mg(3), one Si(2), one Si(3), and three equivalent Si(1) atoms. There are a spread of Mg(2)-Mg(2) bond distances ranging from 3.02-3.12 Å. There are a spread of Mg(2)-Mg(3) bond distances ranging from 2.92-3.01 Å. The Mg(2)-Si(2) bond length is 2.92 Å. The Mg(2)-Si(3) bond length is 2.89 Å. There are a spread of Mg(2)-Si(1) bond distances ranging from 2.91-3.04 Å. In the third Mg site, Mg(3) is bonded to two equivalent Mg(1), six equivalent Mg(2), one Si(2), and three equivalent Si(1) atoms to form distorted MgMg8Si4 cuboctahedra that share corners with two equivalent Mg(1)Mg4Si6 cuboctahedra, corners with nine equivalent Mg(3)Mg8Si4 cuboctahedra, edges with four equivalent Mg(1)Mg4Si6 cuboctahedra, faces with two equivalent Mg(1)Mg4Si6 cuboctahedra, and faces with three equivalent Mg(3)Mg8Si4 cuboctahedra. The Mg(3)-Si(2) bond length is 3.01 Å. There is one shorter (2.95 Å) and two longer (3.07 Å) Mg(3)-Si(1) bond lengths. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded in a 11-coordinate geometry to two equivalent Mg(1), three equivalent Mg(3), and six equivalent Mg(2) atoms. In the second Si site, Si(2) is bonded in a distorted q6 geometry to two equivalent Mg(3), four equivalent Mg(1), and four equivalent Mg(2) atoms. In the third Si site, Si(3) is bonded in a distorted body-centered cubic geometry to four equivalent Mg(1) and four equivalent Mg(2) atoms.