Mg9Si5 crystallizes in the trigonal R32 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Si(2), one Si(3), one Si(4), and one Si(5) atom to form a mixture of distorted corner and edge-sharing MgSi4 tetrahedra. In the second Mg site, Mg(2) is bonded to two equivalent Si(2) and two equivalent Si(3) atoms to form a mixture of distorted corner and edge-sharing MgSi4 tetrahedra. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one Si(2), one Si(4), one Si(5), and two equivalent Si(1) atoms. In the fourth Mg site, Mg(4) is bonded to two equivalent Si(1) and two equivalent Si(5) atoms to form a mixture of distorted corner and edge-sharing MgSi4 tetrahedra. There are five inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to three equivalent Mg(4) and six equivalent Mg(3) atoms. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to three equivalent Mg(1), three equivalent Mg(2), and three equivalent Mg(3) atoms. In the third Si site, Si(3) is bonded in a 7-coordinate geometry to three equivalent Mg(1), three equivalent Mg(2), and one Si(4) atom. In the fourth Si site, Si(4) is bonded in a 7-coordinate geometry to three equivalent Mg(1), three equivalent Mg(3), and one Si(3) atom. In the fifth Si site, Si(5) is bonded in a 9-coordinate geometry to three equivalent Mg(1), three equivalent Mg(3), and three equivalent Mg(4) atoms.
Mg9Si5 crystallizes in the trigonal R32 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Si(2), one Si(3), one Si(4), and one Si(5) atom to form a mixture of distorted corner and edge-sharing MgSi4 tetrahedra. The Mg(1)-Si(2) bond length is 2.78 Å. The Mg(1)-Si(3) bond length is 2.81 Å. The Mg(1)-Si(4) bond length is 2.65 Å. The Mg(1)-Si(5) bond length is 2.78 Å. In the second Mg site, Mg(2) is bonded to two equivalent Si(2) and two equivalent Si(3) atoms to form a mixture of distorted corner and edge-sharing MgSi4 tetrahedra. Both Mg(2)-Si(2) bond lengths are 2.79 Å. Both Mg(2)-Si(3) bond lengths are 2.73 Å. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one Si(2), one Si(4), one Si(5), and two equivalent Si(1) atoms. The Mg(3)-Si(2) bond length is 2.90 Å. The Mg(3)-Si(4) bond length is 2.82 Å. The Mg(3)-Si(5) bond length is 2.88 Å. There is one shorter (2.94 Å) and one longer (2.99 Å) Mg(3)-Si(1) bond length. In the fourth Mg site, Mg(4) is bonded to two equivalent Si(1) and two equivalent Si(5) atoms to form a mixture of distorted corner and edge-sharing MgSi4 tetrahedra. Both Mg(4)-Si(1) bond lengths are 2.72 Å. Both Mg(4)-Si(5) bond lengths are 2.76 Å. There are five inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to three equivalent Mg(4) and six equivalent Mg(3) atoms. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to three equivalent Mg(1), three equivalent Mg(2), and three equivalent Mg(3) atoms. In the third Si site, Si(3) is bonded in a 7-coordinate geometry to three equivalent Mg(1), three equivalent Mg(2), and one Si(4) atom. The Si(3)-Si(4) bond length is 2.50 Å. In the fourth Si site, Si(4) is bonded in a 7-coordinate geometry to three equivalent Mg(1), three equivalent Mg(3), and one Si(3) atom. In the fifth Si site, Si(5) is bonded in a 9-coordinate geometry to three equivalent Mg(1), three equivalent Mg(3), and three equivalent Mg(4) atoms.