Na3Sr2Ti3(Si2O9)2 crystallizes in the monoclinic C2/m space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms to form NaO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Ti(1)O5 trigonal bipyramids, edges with two equivalent Na(1)O6 octahedra, edges with two equivalent Na(2)O6 octahedra, and edges with two equivalent Ti(2)O6 octahedra. In the second Na site, Na(2) is bonded to two equivalent O(4) and four equivalent O(2) atoms to form NaO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, corners with two equivalent Ti(1)O5 trigonal bipyramids, edges with two equivalent Ti(2)O6 octahedra, and edges with four equivalent Na(1)O6 octahedra. Sr(1) is bonded in a 11-coordinate geometry to one O(5), two equivalent O(6), four equivalent O(1), and four equivalent O(3) atoms. There are two inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to one O(4), two equivalent O(1), and two equivalent O(3) atoms to form distorted TiO5 trigonal bipyramids that share  a cornercorner with one Na(2)O6 octahedra, corners with two equivalent Na(1)O6 octahedra, and corners with four equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 56-58°. In the second Ti site, Ti(2) is bonded to two equivalent O(5) and four equivalent O(2) atoms to form TiO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, edges with two equivalent Na(2)O6 octahedra, and edges with four equivalent Na(1)O6 octahedra. Si(1) is bonded to one O(1), one O(2), one O(3), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one Na(1)O6 octahedra,  a cornercorner with one Na(2)O6 octahedra,  a cornercorner with one Ti(2)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Ti(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 54-64°. There are six inequivalent O sites. In the first O site, O(3) is bonded in a 2-coordinate geometry to two equivalent Sr(1), one Ti(1), and one Si(1) atom. In the second O site, O(4) is bonded to one Na(2), two equivalent Na(1), and one Ti(1) atom to form a mixture of distorted edge and corner-sharing ONa3Ti trigonal pyramids. In the third O site, O(5) is bonded in a 3-coordinate geometry to two equivalent Na(1), one Sr(1), and one Ti(2) atom. In the fourth O site, O(6) is bonded in a 2-coordinate geometry to two equivalent Sr(1) and two equivalent Si(1) atoms. In the fifth O site, O(1) is bonded in a 2-coordinate geometry to two equivalent Sr(1), one Ti(1), and one Si(1) atom. In the sixth O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Na(1), one Na(2), one Ti(2), and one Si(1) atom.
Na3Sr2Ti3(Si2O9)2 crystallizes in the monoclinic C2/m space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms to form NaO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Ti(1)O5 trigonal bipyramids, edges with two equivalent Na(1)O6 octahedra, edges with two equivalent Na(2)O6 octahedra, and edges with two equivalent Ti(2)O6 octahedra. Both Na(1)-O(2) bond lengths are 2.28 Å. Both Na(1)-O(4) bond lengths are 2.37 Å. Both Na(1)-O(5) bond lengths are 2.40 Å. In the second Na site, Na(2) is bonded to two equivalent O(4) and four equivalent O(2) atoms to form NaO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, corners with two equivalent Ti(1)O5 trigonal bipyramids, edges with two equivalent Ti(2)O6 octahedra, and edges with four equivalent Na(1)O6 octahedra. Both Na(2)-O(4) bond lengths are 2.30 Å. All Na(2)-O(2) bond lengths are 2.52 Å. Sr(1) is bonded in a 11-coordinate geometry to one O(5), two equivalent O(6), four equivalent O(1), and four equivalent O(3) atoms. The Sr(1)-O(5) bond length is 3.20 Å. There is one shorter (2.82 Å) and one longer (2.85 Å) Sr(1)-O(6) bond length. There are two shorter (2.74 Å) and two longer (2.89 Å) Sr(1)-O(1) bond lengths. There are two shorter (2.72 Å) and two longer (2.83 Å) Sr(1)-O(3) bond lengths. There are two inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to one O(4), two equivalent O(1), and two equivalent O(3) atoms to form distorted TiO5 trigonal bipyramids that share  a cornercorner with one Na(2)O6 octahedra, corners with two equivalent Na(1)O6 octahedra, and corners with four equivalent Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 56-58°. The Ti(1)-O(4) bond length is 1.68 Å. Both Ti(1)-O(1) bond lengths are 2.00 Å. Both Ti(1)-O(3) bond lengths are 2.01 Å. In the second Ti site, Ti(2) is bonded to two equivalent O(5) and four equivalent O(2) atoms to form TiO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra, edges with two equivalent Na(2)O6 octahedra, and edges with four equivalent Na(1)O6 octahedra. Both Ti(2)-O(5) bond lengths are 1.85 Å. All Ti(2)-O(2) bond lengths are 2.07 Å. Si(1) is bonded to one O(1), one O(2), one O(3), and one O(6) atom to form SiO4 tetrahedra that share  a cornercorner with one Na(1)O6 octahedra,  a cornercorner with one Na(2)O6 octahedra,  a cornercorner with one Ti(2)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Ti(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 54-64°. The Si(1)-O(1) bond length is 1.65 Å. The Si(1)-O(2) bond length is 1.61 Å. The Si(1)-O(3) bond length is 1.64 Å. The Si(1)-O(6) bond length is 1.68 Å. There are six inequivalent O sites. In the first O site, O(3) is bonded in a 2-coordinate geometry to two equivalent Sr(1), one Ti(1), and one Si(1) atom. In the second O site, O(4) is bonded to one Na(2), two equivalent Na(1), and one Ti(1) atom to form a mixture of distorted edge and corner-sharing ONa3Ti trigonal pyramids. In the third O site, O(5) is bonded in a 3-coordinate geometry to two equivalent Na(1), one Sr(1), and one Ti(2) atom. In the fourth O site, O(6) is bonded in a 2-coordinate geometry to two equivalent Sr(1) and two equivalent Si(1) atoms. In the fifth O site, O(1) is bonded in a 2-coordinate geometry to two equivalent Sr(1), one Ti(1), and one Si(1) atom. In the sixth O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Na(1), one Na(2), one Ti(2), and one Si(1) atom.