Mg5Si6 crystallizes in the triclinic P-1 space group. There are five inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a distorted rectangular see-saw-like geometry to one Si(1), one Si(2), one Si(3), and one Si(4) atom. In the second Mg site, Mg(2) is bonded in a 7-coordinate geometry to two equivalent Si(1), two equivalent Si(5), and three equivalent Si(3) atoms. In the third Mg site, Mg(3) is bonded in a 6-coordinate geometry to two equivalent Si(2), three equivalent Si(4), and three equivalent Si(6) atoms. In the fourth Mg site, Mg(4) is bonded in a 6-coordinate geometry to one Si(5), two equivalent Si(1), two equivalent Si(2), and two equivalent Si(3) atoms. In the fifth Mg site, Mg(5) is bonded in a 7-coordinate geometry to two equivalent Si(1), two equivalent Si(2), two equivalent Si(4), and two equivalent Si(6) atoms. There are six inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to one Mg(1), two equivalent Mg(2), two equivalent Mg(4), two equivalent Mg(5), one Si(2), and one Si(5) atom. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to one Mg(1), two equivalent Mg(3), two equivalent Mg(4), two equivalent Mg(5), one Si(1), and one Si(6) atom. In the third Si site, Si(3) is bonded in a 8-coordinate geometry to one Mg(1), two equivalent Mg(4), three equivalent Mg(2), one Si(3), and one Si(5) atom. In the fourth Si site, Si(4) is bonded in a 9-coordinate geometry to one Mg(1), two equivalent Mg(5), three equivalent Mg(3), and three equivalent Si(6) atoms. In the fifth Si site, Si(5) is bonded to one Mg(4), two equivalent Mg(2), one Si(1), one Si(3), and two equivalent Si(5) atoms to form a mixture of distorted corner and edge-sharing SiMg3Si4 hexagonal pyramids. In the sixth Si site, Si(6) is bonded in a 9-coordinate geometry to two equivalent Mg(5), three equivalent Mg(3), one Si(2), and three equivalent Si(4) atoms.
Mg5Si6 crystallizes in the triclinic P-1 space group. There are five inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a distorted rectangular see-saw-like geometry to one Si(1), one Si(2), one Si(3), and one Si(4) atom. The Mg(1)-Si(1) bond length is 2.74 Å. The Mg(1)-Si(2) bond length is 2.70 Å. The Mg(1)-Si(3) bond length is 2.84 Å. The Mg(1)-Si(4) bond length is 2.81 Å. In the second Mg site, Mg(2) is bonded in a 7-coordinate geometry to two equivalent Si(1), two equivalent Si(5), and three equivalent Si(3) atoms. There is one shorter (2.82 Å) and one longer (3.03 Å) Mg(2)-Si(1) bond length. There is one shorter (2.88 Å) and one longer (2.99 Å) Mg(2)-Si(5) bond length. There are a spread of Mg(2)-Si(3) bond distances ranging from 2.80-2.97 Å. In the third Mg site, Mg(3) is bonded in a 6-coordinate geometry to two equivalent Si(2), three equivalent Si(4), and three equivalent Si(6) atoms. There is one shorter (2.83 Å) and one longer (2.86 Å) Mg(3)-Si(2) bond length. There are a spread of Mg(3)-Si(4) bond distances ranging from 2.90-3.09 Å. There are a spread of Mg(3)-Si(6) bond distances ranging from 3.03-3.15 Å. In the fourth Mg site, Mg(4) is bonded in a 6-coordinate geometry to one Si(5), two equivalent Si(1), two equivalent Si(2), and two equivalent Si(3) atoms. The Mg(4)-Si(5) bond length is 2.77 Å. There is one shorter (2.87 Å) and one longer (3.17 Å) Mg(4)-Si(1) bond length. There is one shorter (2.77 Å) and one longer (2.80 Å) Mg(4)-Si(2) bond length. There is one shorter (2.76 Å) and one longer (2.94 Å) Mg(4)-Si(3) bond length. In the fifth Mg site, Mg(5) is bonded in a 7-coordinate geometry to two equivalent Si(1), two equivalent Si(2), two equivalent Si(4), and two equivalent Si(6) atoms. There is one shorter (2.82 Å) and one longer (2.91 Å) Mg(5)-Si(1) bond length. There is one shorter (2.86 Å) and one longer (3.09 Å) Mg(5)-Si(2) bond length. There is one shorter (2.92 Å) and one longer (3.08 Å) Mg(5)-Si(4) bond length. There is one shorter (2.94 Å) and one longer (3.21 Å) Mg(5)-Si(6) bond length. There are six inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to one Mg(1), two equivalent Mg(2), two equivalent Mg(4), two equivalent Mg(5), one Si(2), and one Si(5) atom. The Si(1)-Si(2) bond length is 2.43 Å. The Si(1)-Si(5) bond length is 2.46 Å. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to one Mg(1), two equivalent Mg(3), two equivalent Mg(4), two equivalent Mg(5), one Si(1), and one Si(6) atom. The Si(2)-Si(6) bond length is 2.38 Å. In the third Si site, Si(3) is bonded in a 8-coordinate geometry to one Mg(1), two equivalent Mg(4), three equivalent Mg(2), one Si(3), and one Si(5) atom. The Si(3)-Si(3) bond length is 2.41 Å. The Si(3)-Si(5) bond length is 2.50 Å. In the fourth Si site, Si(4) is bonded in a 9-coordinate geometry to one Mg(1), two equivalent Mg(5), three equivalent Mg(3), and three equivalent Si(6) atoms. There are two shorter (2.50 Å) and one longer (2.53 Å) Si(4)-Si(6) bond length. In the fifth Si site, Si(5) is bonded to one Mg(4), two equivalent Mg(2), one Si(1), one Si(3), and two equivalent Si(5) atoms to form a mixture of distorted corner and edge-sharing SiMg3Si4 hexagonal pyramids. There is one shorter (2.34 Å) and one longer (2.60 Å) Si(5)-Si(5) bond length. In the sixth Si site, Si(6) is bonded in a 9-coordinate geometry to two equivalent Mg(5), three equivalent Mg(3), one Si(2), and three equivalent Si(4) atoms.