Mg9Si5 crystallizes in the hexagonal P6_3/m space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Si(1), one Si(2), and two equivalent Si(3) atoms to form a mixture of distorted edge and corner-sharing MgSi4 tetrahedra. In the second Mg site, Mg(2) is bonded in a 5-coordinate geometry to one Si(2), two equivalent Si(1), and two equivalent Si(3) atoms. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded in a 7-coordinate geometry to three equivalent Mg(1), three equivalent Mg(2), and one Si(1) atom. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to three equivalent Mg(2) and six equivalent Mg(1) atoms. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to three equivalent Mg(2) and six equivalent Mg(1) atoms.
Mg9Si5 crystallizes in the hexagonal P6_3/m space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Si(1), one Si(2), and two equivalent Si(3) atoms to form a mixture of distorted edge and corner-sharing MgSi4 tetrahedra. The Mg(1)-Si(1) bond length is 2.65 Å. The Mg(1)-Si(2) bond length is 2.75 Å. There is one shorter (2.75 Å) and one longer (2.78 Å) Mg(1)-Si(3) bond length. In the second Mg site, Mg(2) is bonded in a 5-coordinate geometry to one Si(2), two equivalent Si(1), and two equivalent Si(3) atoms. The Mg(2)-Si(2) bond length is 2.85 Å. Both Mg(2)-Si(1) bond lengths are 2.91 Å. Both Mg(2)-Si(3) bond lengths are 2.91 Å. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded in a 7-coordinate geometry to three equivalent Mg(1), three equivalent Mg(2), and one Si(1) atom. The Si(1)-Si(1) bond length is 2.47 Å. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to three equivalent Mg(2) and six equivalent Mg(1) atoms. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to three equivalent Mg(2) and six equivalent Mg(1) atoms.