Mg5Si6 crystallizes in the monoclinic C2/m space group. There are five inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 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 6-coordinate geometry to one Si(6), two equivalent Si(1), and three equivalent Si(3) atoms. In the third Mg site, Mg(3) is bonded in a 8-coordinate geometry to two equivalent Si(2), two equivalent Si(4), and four equivalent Si(5) atoms. In the fourth Mg site, Mg(4) is bonded in a 8-coordinate geometry to two equivalent Si(1), two equivalent Si(2), two equivalent Si(3), and two equivalent Si(6) atoms. In the fifth Mg site, Mg(5) is bonded in a 8-coordinate geometry to two equivalent Si(1), two equivalent Si(2), two equivalent Si(4), and two equivalent Si(5) 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(6) 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(5) atom. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to one Mg(1), two equivalent Mg(4), three equivalent Mg(2), and three equivalent Si(6) atoms. In the fourth Si site, Si(4) is bonded in a 7-coordinate geometry to one Mg(1), two equivalent Mg(3), two equivalent Mg(5), one Si(4), and one Si(5) atom. In the fifth Si site, Si(5) is bonded in a 9-coordinate geometry to two equivalent Mg(5), four equivalent Mg(3), one Si(2), one Si(4), and one Si(5) atom. In the sixth Si site, Si(6) is bonded in a 7-coordinate geometry to one Mg(2), two equivalent Mg(4), one Si(1), and three equivalent Si(3) atoms.
Mg5Si6 crystallizes in the monoclinic C2/m space group. There are five inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 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 2.67 Å. The Mg(1)-Si(2) bond length is 2.64 Å. The Mg(1)-Si(3) bond length is 2.79 Å. The Mg(1)-Si(4) bond length is 2.73 Å. In the second Mg site, Mg(2) is bonded in a 6-coordinate geometry to one Si(6), two equivalent Si(1), and three equivalent Si(3) atoms. The Mg(2)-Si(6) bond length is 3.04 Å. Both Mg(2)-Si(1) bond lengths are 2.81 Å. There is one shorter (2.94 Å) and two longer (2.99 Å) Mg(2)-Si(3) bond lengths. In the third Mg site, Mg(3) is bonded in a 8-coordinate geometry to two equivalent Si(2), two equivalent Si(4), and four equivalent Si(5) atoms. Both Mg(3)-Si(2) bond lengths are 2.91 Å. Both Mg(3)-Si(4) bond lengths are 2.92 Å. There are two shorter (2.89 Å) and two longer (3.15 Å) Mg(3)-Si(5) bond lengths. In the fourth Mg site, Mg(4) is bonded in a 8-coordinate geometry to two equivalent Si(1), two equivalent Si(2), two equivalent Si(3), and two equivalent Si(6) atoms. Both Mg(4)-Si(1) bond lengths are 2.96 Å. Both Mg(4)-Si(2) bond lengths are 2.80 Å. Both Mg(4)-Si(3) bond lengths are 3.08 Å. Both Mg(4)-Si(6) bond lengths are 3.04 Å. In the fifth Mg site, Mg(5) is bonded in a 8-coordinate geometry to two equivalent Si(1), two equivalent Si(2), two equivalent Si(4), and two equivalent Si(5) atoms. Both Mg(5)-Si(1) bond lengths are 2.99 Å. Both Mg(5)-Si(2) bond lengths are 3.06 Å. Both Mg(5)-Si(4) bond lengths are 2.76 Å. Both Mg(5)-Si(5) bond lengths are 2.83 Å. 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(6) atom. The Si(1)-Si(2) bond length is 2.38 Å. The Si(1)-Si(6) bond length is 2.41 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(5) atom. The Si(2)-Si(5) bond length is 2.40 Å. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to one Mg(1), two equivalent Mg(4), three equivalent Mg(2), and three equivalent Si(6) atoms. There are two shorter (2.47 Å) and one longer (2.50 Å) Si(3)-Si(6) bond length. In the fourth Si site, Si(4) is bonded in a 7-coordinate geometry to one Mg(1), two equivalent Mg(3), two equivalent Mg(5), one Si(4), and one Si(5) atom. The Si(4)-Si(4) bond length is 2.42 Å. The Si(4)-Si(5) bond length is 2.52 Å. In the fifth Si site, Si(5) is bonded in a 9-coordinate geometry to two equivalent Mg(5), four equivalent Mg(3), one Si(2), one Si(4), and one Si(5) atom. The Si(5)-Si(5) bond length is 2.56 Å. In the sixth Si site, Si(6) is bonded in a 7-coordinate geometry to one Mg(2), two equivalent Mg(4), one Si(1), and three equivalent Si(3) atoms.