Mg5Si6 crystallizes in the orthorhombic Ama2 space group. There are five inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 5-coordinate geometry to two equivalent Mg(5), one Si(2), one Si(3), one Si(4), and two equivalent Si(1) atoms. In the second Mg site, Mg(2) is bonded in a 8-coordinate geometry to two equivalent Mg(5), two equivalent Si(1), three equivalent Si(3), and three equivalent Si(5) atoms. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one Mg(5), one Si(6), two equivalent Si(1), two equivalent Si(2), and two equivalent Si(4) atoms. In the fourth Mg site, Mg(4) is bonded in a 8-coordinate geometry to two equivalent Si(2), three equivalent Si(4), and three equivalent Si(6) atoms. In the fifth Mg site, Mg(5) is bonded in a 13-coordinate geometry to one Mg(3), two equivalent Mg(1), two equivalent Mg(2), two equivalent Si(1), three equivalent Si(3), and three equivalent Si(5) atoms. There are six inequivalent Si sites. In the first Si site, Si(1) is bonded in a 10-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(5), one Si(2), and one Si(5) atom. In the second Si site, Si(2) is bonded in a 8-coordinate geometry to one Mg(1), two equivalent Mg(3), two equivalent Mg(4), one Si(1), one Si(4), and one Si(6) atom. In the third Si site, Si(3) is bonded in a 10-coordinate geometry to one Mg(1), three equivalent Mg(2), three equivalent Mg(5), one Si(5), and two equivalent Si(3) atoms. In the fourth Si site, Si(4) is bonded in a 10-coordinate geometry to one Mg(1), two equivalent Mg(3), three equivalent Mg(4), one Si(2), and three equivalent Si(6) atoms. In the fifth Si site, Si(5) is bonded in a 10-coordinate geometry to three equivalent Mg(2), three equivalent Mg(5), one Si(1), one Si(3), and two equivalent Si(5) atoms. In the sixth Si site, Si(6) is bonded to one Mg(3), three equivalent Mg(4), one Si(2), and three equivalent Si(4) atoms to form distorted edge-sharing SiMg4Si4 hexagonal bipyramids.
Mg5Si6 crystallizes in the orthorhombic Ama2 space group. There are five inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 5-coordinate geometry to two equivalent Mg(5), one Si(2), one Si(3), one Si(4), and two equivalent Si(1) atoms. Both Mg(1)-Mg(5) bond lengths are 3.11 Å. The Mg(1)-Si(2) bond length is 3.07 Å. The Mg(1)-Si(3) bond length is 2.99 Å. The Mg(1)-Si(4) bond length is 3.01 Å. There is one shorter (2.97 Å) and one longer (3.12 Å) Mg(1)-Si(1) bond length. In the second Mg site, Mg(2) is bonded in a 8-coordinate geometry to two equivalent Mg(5), two equivalent Si(1), three equivalent Si(3), and three equivalent Si(5) atoms. There is one shorter (3.01 Å) and one longer (3.04 Å) Mg(2)-Mg(5) bond length. Both Mg(2)-Si(1) bond lengths are 3.02 Å. There is one shorter (2.83 Å) and two longer (2.94 Å) Mg(2)-Si(3) bond lengths. There is one shorter (2.80 Å) and two longer (3.05 Å) Mg(2)-Si(5) bond lengths. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one Mg(5), one Si(6), two equivalent Si(1), two equivalent Si(2), and two equivalent Si(4) atoms. The Mg(3)-Mg(5) bond length is 3.37 Å. The Mg(3)-Si(6) bond length is 2.80 Å. Both Mg(3)-Si(1) bond lengths are 2.83 Å. Both Mg(3)-Si(2) bond lengths are 2.81 Å. Both Mg(3)-Si(4) bond lengths are 3.05 Å. In the fourth Mg site, Mg(4) is bonded in a 8-coordinate geometry to two equivalent Si(2), three equivalent Si(4), and three equivalent Si(6) atoms. Both Mg(4)-Si(2) bond lengths are 2.77 Å. There is one shorter (2.89 Å) and two longer (3.01 Å) Mg(4)-Si(4) bond lengths. There are two shorter (2.85 Å) and one longer (3.06 Å) Mg(4)-Si(6) bond length. In the fifth Mg site, Mg(5) is bonded in a 13-coordinate geometry to one Mg(3), two equivalent Mg(1), two equivalent Mg(2), two equivalent Si(1), three equivalent Si(3), and three equivalent Si(5) atoms. Both Mg(5)-Si(1) bond lengths are 2.84 Å. There are two shorter (2.96 Å) and one longer (3.08 Å) Mg(5)-Si(3) bond length. There is one shorter (2.92 Å) and two longer (2.99 Å) Mg(5)-Si(5) bond lengths. There are six inequivalent Si sites. In the first Si site, Si(1) is bonded in a 10-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), two equivalent Mg(5), one Si(2), and one Si(5) atom. The Si(1)-Si(2) bond length is 2.61 Å. The Si(1)-Si(5) bond length is 2.51 Å. In the second Si site, Si(2) is bonded in a 8-coordinate geometry to one Mg(1), two equivalent Mg(3), two equivalent Mg(4), one Si(1), one Si(4), and one Si(6) atom. The Si(2)-Si(4) bond length is 2.56 Å. The Si(2)-Si(6) bond length is 2.42 Å. In the third Si site, Si(3) is bonded in a 10-coordinate geometry to one Mg(1), three equivalent Mg(2), three equivalent Mg(5), one Si(5), and two equivalent Si(3) atoms. The Si(3)-Si(5) bond length is 2.50 Å. Both Si(3)-Si(3) bond lengths are 2.60 Å. In the fourth Si site, Si(4) is bonded in a 10-coordinate geometry to one Mg(1), two equivalent Mg(3), three equivalent Mg(4), one Si(2), and three equivalent Si(6) atoms. There are two shorter (2.53 Å) and one longer (2.55 Å) Si(4)-Si(6) bond length. In the fifth Si site, Si(5) is bonded in a 10-coordinate geometry to three equivalent Mg(2), three equivalent Mg(5), one Si(1), one Si(3), and two equivalent Si(5) atoms. Both Si(5)-Si(5) bond lengths are 2.48 Å. In the sixth Si site, Si(6) is bonded to one Mg(3), three equivalent Mg(4), one Si(2), and three equivalent Si(4) atoms to form distorted edge-sharing SiMg4Si4 hexagonal bipyramids.