Mg5Si6 crystallizes in the monoclinic P2/m space group. There are six inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a distorted linear geometry to two equivalent Si(1) atoms. In the second Mg site, Mg(2) is bonded in a square co-planar geometry to two equivalent Si(2) and two equivalent Si(4) atoms. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one Si(3), two equivalent Si(2), two equivalent Si(4), and two equivalent Si(5) atoms. In the fourth Mg site, Mg(4) is bonded in a 7-coordinate geometry to one Si(3), two equivalent Si(5), and four equivalent Si(1) atoms. In the fifth Mg site, Mg(5) is bonded in a 6-coordinate geometry to two equivalent Si(4), two equivalent Si(6), and four equivalent Si(2) atoms. In the sixth Mg site, Mg(6) is bonded in a 9-coordinate geometry to one Si(3), two equivalent Si(1), two equivalent Si(4), two equivalent Si(5), 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(6), four equivalent Mg(4), one Si(1), and one Si(5) atom. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to one Mg(2), two equivalent Mg(3), four equivalent Mg(5), one Si(2), and one Si(6) atom. In the third Si site, Si(3) is bonded in a 7-coordinate geometry to one Mg(3), one Mg(4), one Mg(6), two equivalent Si(5), and two equivalent Si(6) atoms. In the fourth Si site, Si(4) is bonded in a 9-coordinate geometry to one Mg(2), two equivalent Mg(3), two equivalent Mg(5), two equivalent Mg(6), one Si(5), and one Si(6) atom. In the fifth Si site, Si(5) is bonded in a 10-coordinate geometry to two equivalent Mg(3), two equivalent Mg(4), two equivalent Mg(6), one Si(1), one Si(4), and two equivalent Si(3) atoms. In the sixth Si site, Si(6) is bonded in a 8-coordinate geometry to two equivalent Mg(5), two equivalent Mg(6), one Si(2), one Si(4), and two equivalent Si(3) atoms.
Mg5Si6 crystallizes in the monoclinic P2/m space group. There are six inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a distorted linear geometry to two equivalent Si(1) atoms. Both Mg(1)-Si(1) bond lengths are 2.81 Å. In the second Mg site, Mg(2) is bonded in a square co-planar geometry to two equivalent Si(2) and two equivalent Si(4) atoms. Both Mg(2)-Si(2) bond lengths are 2.79 Å. Both Mg(2)-Si(4) bond lengths are 2.80 Å. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one Si(3), two equivalent Si(2), two equivalent Si(4), and two equivalent Si(5) atoms. The Mg(3)-Si(3) bond length is 2.74 Å. Both Mg(3)-Si(2) bond lengths are 2.81 Å. Both Mg(3)-Si(4) bond lengths are 2.84 Å. Both Mg(3)-Si(5) bond lengths are 3.14 Å. In the fourth Mg site, Mg(4) is bonded in a 7-coordinate geometry to one Si(3), two equivalent Si(5), and four equivalent Si(1) atoms. The Mg(4)-Si(3) bond length is 2.68 Å. Both Mg(4)-Si(5) bond lengths are 2.89 Å. There are two shorter (2.75 Å) and two longer (2.87 Å) Mg(4)-Si(1) bond lengths. In the fifth Mg site, Mg(5) is bonded in a 6-coordinate geometry to two equivalent Si(4), two equivalent Si(6), and four equivalent Si(2) atoms. Both Mg(5)-Si(4) bond lengths are 2.88 Å. Both Mg(5)-Si(6) bond lengths are 3.12 Å. There are two shorter (2.89 Å) and two longer (2.94 Å) Mg(5)-Si(2) bond lengths. In the sixth Mg site, Mg(6) is bonded in a 9-coordinate geometry to one Si(3), two equivalent Si(1), two equivalent Si(4), two equivalent Si(5), and two equivalent Si(6) atoms. The Mg(6)-Si(3) bond length is 2.73 Å. Both Mg(6)-Si(1) bond lengths are 2.79 Å. Both Mg(6)-Si(4) bond lengths are 2.83 Å. Both Mg(6)-Si(5) bond lengths are 2.97 Å. Both Mg(6)-Si(6) bond lengths are 3.02 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(6), four equivalent Mg(4), one Si(1), and one Si(5) atom. The Si(1)-Si(1) bond length is 2.44 Å. The Si(1)-Si(5) bond length is 2.56 Å. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to one Mg(2), two equivalent Mg(3), four equivalent Mg(5), one Si(2), and one Si(6) atom. The Si(2)-Si(2) bond length is 2.42 Å. The Si(2)-Si(6) bond length is 2.49 Å. In the third Si site, Si(3) is bonded in a 7-coordinate geometry to one Mg(3), one Mg(4), one Mg(6), two equivalent Si(5), and two equivalent Si(6) atoms. Both Si(3)-Si(5) bond lengths are 2.79 Å. Both Si(3)-Si(6) bond lengths are 2.52 Å. In the fourth Si site, Si(4) is bonded in a 9-coordinate geometry to one Mg(2), two equivalent Mg(3), two equivalent Mg(5), two equivalent Mg(6), one Si(5), and one Si(6) atom. The Si(4)-Si(5) bond length is 2.41 Å. The Si(4)-Si(6) bond length is 2.46 Å. In the fifth Si site, Si(5) is bonded in a 10-coordinate geometry to two equivalent Mg(3), two equivalent Mg(4), two equivalent Mg(6), one Si(1), one Si(4), and two equivalent Si(3) atoms. In the sixth Si site, Si(6) is bonded in a 8-coordinate geometry to two equivalent Mg(5), two equivalent Mg(6), one Si(2), one Si(4), and two equivalent Si(3) atoms.