Sr11(MgSi5)2 crystallizes in the monoclinic C2/m space group. There are six inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 7-coordinate geometry to two equivalent Si(2), two equivalent Si(3), and three equivalent Si(1) atoms. In the second Sr site, Sr(2) is bonded in a 5-coordinate geometry to one Si(4), two equivalent Si(2), and two equivalent Si(5) atoms. In the third Sr site, Sr(3) is bonded in a 7-coordinate geometry to one Si(5), two equivalent Si(1), two equivalent Si(3), and two equivalent Si(4) atoms. In the fourth Sr site, Sr(4) is bonded in a 7-coordinate geometry to one Si(2), two equivalent Si(1), and four equivalent Si(4) atoms. In the fifth Sr site, Sr(5) is bonded in a 9-coordinate geometry to three equivalent Mg(1), two equivalent Si(2), two equivalent Si(3), and two equivalent Si(5) atoms. In the sixth Sr site, Sr(6) is bonded in a square co-planar geometry to two equivalent Si(2) and two equivalent Si(5) atoms. Mg(1) is bonded to three equivalent Sr(5), one Si(3), and three equivalent Si(5) atoms to form a mixture of distorted edge and face-sharing MgSr3Si4 tetrahedra. There are five inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to two equivalent Sr(3), two equivalent Sr(4), three equivalent Sr(1), one Si(3), and one Si(4) atom. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to one Sr(4), one Sr(6), two equivalent Sr(1), two equivalent Sr(2), two equivalent Sr(5), and one Si(3) atom. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to two equivalent Sr(1), two equivalent Sr(3), two equivalent Sr(5), one Mg(1), one Si(1), and one Si(2) atom. In the fourth Si site, Si(4) is bonded in a 9-coordinate geometry to one Sr(2), two equivalent Sr(3), four equivalent Sr(4), one Si(1), and one Si(4) atom. In the fifth Si site, Si(5) is bonded in a 9-coordinate geometry to one Sr(3), one Sr(6), two equivalent Sr(2), two equivalent Sr(5), and three equivalent Mg(1) atoms.
Sr11(MgSi5)2 crystallizes in the monoclinic C2/m space group. There are six inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 7-coordinate geometry to two equivalent Si(2), two equivalent Si(3), and three equivalent Si(1) atoms. Both Sr(1)-Si(2) bond lengths are 3.26 Å. Both Sr(1)-Si(3) bond lengths are 3.24 Å. There are two shorter (3.31 Å) and one longer (3.34 Å) Sr(1)-Si(1) bond length. In the second Sr site, Sr(2) is bonded in a 5-coordinate geometry to one Si(4), two equivalent Si(2), and two equivalent Si(5) atoms. The Sr(2)-Si(4) bond length is 3.53 Å. Both Sr(2)-Si(2) bond lengths are 3.36 Å. Both Sr(2)-Si(5) bond lengths are 3.44 Å. In the third Sr site, Sr(3) is bonded in a 7-coordinate geometry to one Si(5), two equivalent Si(1), two equivalent Si(3), and two equivalent Si(4) atoms. The Sr(3)-Si(5) bond length is 3.18 Å. Both Sr(3)-Si(1) bond lengths are 3.31 Å. Both Sr(3)-Si(3) bond lengths are 3.22 Å. Both Sr(3)-Si(4) bond lengths are 3.24 Å. In the fourth Sr site, Sr(4) is bonded in a 7-coordinate geometry to one Si(2), two equivalent Si(1), and four equivalent Si(4) atoms. The Sr(4)-Si(2) bond length is 3.43 Å. Both Sr(4)-Si(1) bond lengths are 3.26 Å. There are two shorter (3.24 Å) and two longer (3.35 Å) Sr(4)-Si(4) bond lengths. In the fifth Sr site, Sr(5) is bonded in a 9-coordinate geometry to three equivalent Mg(1), two equivalent Si(2), two equivalent Si(3), and two equivalent Si(5) atoms. There is one shorter (3.44 Å) and two longer (3.50 Å) Sr(5)-Mg(1) bond lengths. Both Sr(5)-Si(2) bond lengths are 3.42 Å. Both Sr(5)-Si(3) bond lengths are 3.24 Å. Both Sr(5)-Si(5) bond lengths are 3.45 Å. In the sixth Sr site, Sr(6) is bonded in a square co-planar geometry to two equivalent Si(2) and two equivalent Si(5) atoms. Both Sr(6)-Si(2) bond lengths are 3.38 Å. Both Sr(6)-Si(5) bond lengths are 3.35 Å. Mg(1) is bonded to three equivalent Sr(5), one Si(3), and three equivalent Si(5) atoms to form a mixture of distorted edge and face-sharing MgSr3Si4 tetrahedra. The Mg(1)-Si(3) bond length is 2.94 Å. There is one shorter (2.81 Å) and two longer (2.87 Å) Mg(1)-Si(5) bond lengths. There are five inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to two equivalent Sr(3), two equivalent Sr(4), three equivalent Sr(1), one Si(3), and one Si(4) atom. The Si(1)-Si(3) bond length is 2.46 Å. The Si(1)-Si(4) bond length is 2.47 Å. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to one Sr(4), one Sr(6), two equivalent Sr(1), two equivalent Sr(2), two equivalent Sr(5), and one Si(3) atom. The Si(2)-Si(3) bond length is 2.46 Å. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to two equivalent Sr(1), two equivalent Sr(3), two equivalent Sr(5), one Mg(1), one Si(1), and one Si(2) atom. In the fourth Si site, Si(4) is bonded in a 9-coordinate geometry to one Sr(2), two equivalent Sr(3), four equivalent Sr(4), one Si(1), and one Si(4) atom. The Si(4)-Si(4) bond length is 2.47 Å. In the fifth Si site, Si(5) is bonded in a 9-coordinate geometry to one Sr(3), one Sr(6), two equivalent Sr(2), two equivalent Sr(5), and three equivalent Mg(1) atoms.