Na8SnSi6O18 crystallizes in the trigonal R-3m space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a body-centered cubic geometry to four equivalent O(2) and four equivalent O(3) atoms. In the second Na site, Na(2) is bonded to three equivalent O(1) and three equivalent O(3) atoms to form distorted NaO6 octahedra that share corners with six equivalent Si(1)O4 tetrahedra, faces with three equivalent Na(3)O8 hexagonal bipyramids, and  a faceface with one Sn(1)O6 octahedra. In the third Na site, Na(3) is bonded to two equivalent O(2), two equivalent O(3), and four equivalent O(1) atoms to form NaO8 hexagonal bipyramids that share corners with four equivalent Na(3)O8 hexagonal bipyramids, corners with two equivalent Si(1)O4 tetrahedra, edges with two equivalent Sn(1)O6 octahedra, edges with four equivalent Si(1)O4 tetrahedra, and faces with two equivalent Na(2)O6 octahedra. Sn(1) is bonded to six equivalent O(1) atoms to form SnO6 octahedra that share corners with six equivalent Si(1)O4 tetrahedra, edges with six equivalent Na(3)O8 hexagonal bipyramids, and faces with two equivalent Na(2)O6 octahedra. Si(1) is bonded to one O(1), one O(3), and two equivalent O(2) atoms to form SiO4 tetrahedra that share  a cornercorner with one Na(3)O8 hexagonal bipyramid,  a cornercorner with one Sn(1)O6 octahedra, corners with two equivalent Na(2)O6 octahedra, corners with two equivalent Si(1)O4 tetrahedra, and edges with two equivalent Na(3)O8 hexagonal bipyramids. The corner-sharing octahedral tilt angles range from 36-50°. There are three inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one Na(2), two equivalent Na(3), one Sn(1), and one Si(1) atom. In the second O site, O(2) is bonded to one Na(3), two equivalent Na(1), and two equivalent Si(1) atoms to form a mixture of distorted edge and corner-sharing ONa3Si2 trigonal bipyramids. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Na(2), one Na(3), two equivalent Na(1), and one Si(1) atom.
Na8SnSi6O18 crystallizes in the trigonal R-3m space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a body-centered cubic geometry to four equivalent O(2) and four equivalent O(3) atoms. All Na(1)-O(2) bond lengths are 2.68 Å. All Na(1)-O(3) bond lengths are 2.64 Å. In the second Na site, Na(2) is bonded to three equivalent O(1) and three equivalent O(3) atoms to form distorted NaO6 octahedra that share corners with six equivalent Si(1)O4 tetrahedra, faces with three equivalent Na(3)O8 hexagonal bipyramids, and  a faceface with one Sn(1)O6 octahedra. All Na(2)-O(1) bond lengths are 2.52 Å. All Na(2)-O(3) bond lengths are 2.28 Å. In the third Na site, Na(3) is bonded to two equivalent O(2), two equivalent O(3), and four equivalent O(1) atoms to form NaO8 hexagonal bipyramids that share corners with four equivalent Na(3)O8 hexagonal bipyramids, corners with two equivalent Si(1)O4 tetrahedra, edges with two equivalent Sn(1)O6 octahedra, edges with four equivalent Si(1)O4 tetrahedra, and faces with two equivalent Na(2)O6 octahedra. Both Na(3)-O(2) bond lengths are 2.50 Å. Both Na(3)-O(3) bond lengths are 2.32 Å. All Na(3)-O(1) bond lengths are 2.73 Å. Sn(1) is bonded to six equivalent O(1) atoms to form SnO6 octahedra that share corners with six equivalent Si(1)O4 tetrahedra, edges with six equivalent Na(3)O8 hexagonal bipyramids, and faces with two equivalent Na(2)O6 octahedra. All Sn(1)-O(1) bond lengths are 2.08 Å. Si(1) is bonded to one O(1), one O(3), and two equivalent O(2) atoms to form SiO4 tetrahedra that share  a cornercorner with one Na(3)O8 hexagonal bipyramid,  a cornercorner with one Sn(1)O6 octahedra, corners with two equivalent Na(2)O6 octahedra, corners with two equivalent Si(1)O4 tetrahedra, and edges with two equivalent Na(3)O8 hexagonal bipyramids. The corner-sharing octahedral tilt angles range from 36-50°. The Si(1)-O(1) bond length is 1.64 Å. The Si(1)-O(3) bond length is 1.59 Å. Both Si(1)-O(2) bond lengths are 1.67 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one Na(2), two equivalent Na(3), one Sn(1), and one Si(1) atom. In the second O site, O(2) is bonded to one Na(3), two equivalent Na(1), and two equivalent Si(1) atoms to form a mixture of distorted edge and corner-sharing ONa3Si2 trigonal bipyramids. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Na(2), one Na(3), two equivalent Na(1), and one Si(1) atom.