La4Ti9(Si2O15)2 crystallizes in the monoclinic C2/m space group. There are two inequivalent La sites. In the first La site, La(1) is bonded in a 8-coordinate geometry to one O(11), one O(3), two equivalent O(1), two equivalent O(2), and two equivalent O(9) atoms. In the second La site, La(2) is bonded in a 8-coordinate geometry to one O(11), one O(5), two equivalent O(1), two equivalent O(2), and two equivalent O(9) atoms. There are four inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to one O(4), one O(5), two equivalent O(1), and two equivalent O(8) atoms to form a mixture of distorted corner and edge-sharing TiO6 octahedra. The corner-sharing octahedral tilt angles range from 47-49°. In the second Ti site, Ti(2) is bonded to one O(2), one O(3), one O(4), one O(5), one O(7), and one O(8) atom to form TiO6 octahedra that share corners with three equivalent Ti(1)O6 octahedra, corners with three equivalent Ti(3)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and edges with two equivalent Ti(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 47-49°. In the third Ti site, Ti(3) is bonded to one O(3), one O(7), two equivalent O(1), and two equivalent O(8) atoms to form a mixture of distorted corner and edge-sharing TiO6 octahedra. The corner-sharing octahedral tilt angles range from 47-49°. In the fourth Ti site, Ti(4) is bonded to two equivalent O(10) and four equivalent O(9) atoms to form TiO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra and corners with four equivalent Si(2)O4 tetrahedra. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(10), one O(6), and two equivalent O(2) atoms to form SiO4 tetrahedra that share  a cornercorner with one Ti(4)O6 octahedra, corners with two equivalent Ti(2)O6 octahedra, and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 28-54°. In the second Si site, Si(2) is bonded to one O(11), one O(6), and two equivalent O(9) atoms to form SiO4 tetrahedra that share corners with two equivalent Ti(4)O6 octahedra and  a cornercorner with one Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 43°. There are eleven inequivalent O sites. In the first O site, O(1) is bonded in a distorted see-saw-like geometry to one La(1), one La(2), one Ti(1), and one Ti(3) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to one La(1), one La(2), one Ti(2), and one Si(1) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to one La(1), one Ti(3), and two equivalent Ti(2) atoms. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Ti(1) and two equivalent Ti(2) atoms. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one La(2), one Ti(1), and two equivalent Ti(2) atoms. In the sixth O site, O(6) is bonded in a linear geometry to one Si(1) and one Si(2) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to one Ti(3) and two equivalent Ti(2) atoms. In the eighth O site, O(8) is bonded in a distorted trigonal planar geometry to one Ti(1), one Ti(2), and one Ti(3) atom. In the ninth O site, O(9) is bonded in a 4-coordinate geometry to one La(1), one La(2), one Ti(4), and one Si(2) atom. In the tenth O site, O(10) is bonded in a bent 150 degrees geometry to one Ti(4) and one Si(1) atom. In the eleventh O site, O(11) is bonded in a distorted single-bond geometry to one La(1), one La(2), and one Si(2) atom.
La4Ti9(Si2O15)2 crystallizes in the monoclinic C2/m space group. There are two inequivalent La sites. In the first La site, La(1) is bonded in a 8-coordinate geometry to one O(11), one O(3), two equivalent O(1), two equivalent O(2), and two equivalent O(9) atoms. The La(1)-O(11) bond length is 2.54 Å. The La(1)-O(3) bond length is 2.62 Å. Both La(1)-O(1) bond lengths are 2.49 Å. Both La(1)-O(2) bond lengths are 2.57 Å. Both La(1)-O(9) bond lengths are 2.69 Å. In the second La site, La(2) is bonded in a 8-coordinate geometry to one O(11), one O(5), two equivalent O(1), two equivalent O(2), and two equivalent O(9) atoms. The La(2)-O(11) bond length is 2.53 Å. The La(2)-O(5) bond length is 2.54 Å. Both La(2)-O(1) bond lengths are 2.48 Å. Both La(2)-O(2) bond lengths are 2.55 Å. Both La(2)-O(9) bond lengths are 2.76 Å. There are four inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to one O(4), one O(5), two equivalent O(1), and two equivalent O(8) atoms to form a mixture of distorted corner and edge-sharing TiO6 octahedra. The corner-sharing octahedral tilt angles range from 47-49°. The Ti(1)-O(4) bond length is 1.84 Å. The Ti(1)-O(5) bond length is 2.32 Å. Both Ti(1)-O(1) bond lengths are 2.03 Å. Both Ti(1)-O(8) bond lengths are 1.98 Å. In the second Ti site, Ti(2) is bonded to one O(2), one O(3), one O(4), one O(5), one O(7), and one O(8) atom to form TiO6 octahedra that share corners with three equivalent Ti(1)O6 octahedra, corners with three equivalent Ti(3)O6 octahedra,  a cornercorner with one Si(1)O4 tetrahedra, and edges with two equivalent Ti(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 47-49°. The Ti(2)-O(2) bond length is 2.17 Å. The Ti(2)-O(3) bond length is 1.96 Å. The Ti(2)-O(4) bond length is 2.04 Å. The Ti(2)-O(5) bond length is 1.97 Å. The Ti(2)-O(7) bond length is 2.04 Å. The Ti(2)-O(8) bond length is 1.92 Å. In the third Ti site, Ti(3) is bonded to one O(3), one O(7), two equivalent O(1), and two equivalent O(8) atoms to form a mixture of distorted corner and edge-sharing TiO6 octahedra. The corner-sharing octahedral tilt angles range from 47-49°. The Ti(3)-O(3) bond length is 2.32 Å. The Ti(3)-O(7) bond length is 1.84 Å. Both Ti(3)-O(1) bond lengths are 2.02 Å. Both Ti(3)-O(8) bond lengths are 1.98 Å. In the fourth Ti site, Ti(4) is bonded to two equivalent O(10) and four equivalent O(9) atoms to form TiO6 octahedra that share corners with two equivalent Si(1)O4 tetrahedra and corners with four equivalent Si(2)O4 tetrahedra. Both Ti(4)-O(10) bond lengths are 1.94 Å. All Ti(4)-O(9) bond lengths are 2.13 Å. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(10), one O(6), and two equivalent O(2) atoms to form SiO4 tetrahedra that share  a cornercorner with one Ti(4)O6 octahedra, corners with two equivalent Ti(2)O6 octahedra, and  a cornercorner with one Si(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 28-54°. The Si(1)-O(10) bond length is 1.62 Å. The Si(1)-O(6) bond length is 1.63 Å. Both Si(1)-O(2) bond lengths are 1.64 Å. In the second Si site, Si(2) is bonded to one O(11), one O(6), and two equivalent O(9) atoms to form SiO4 tetrahedra that share corners with two equivalent Ti(4)O6 octahedra and  a cornercorner with one Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 43°. The Si(2)-O(11) bond length is 1.61 Å. The Si(2)-O(6) bond length is 1.64 Å. Both Si(2)-O(9) bond lengths are 1.66 Å. There are eleven inequivalent O sites. In the first O site, O(1) is bonded in a distorted see-saw-like geometry to one La(1), one La(2), one Ti(1), and one Ti(3) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to one La(1), one La(2), one Ti(2), and one Si(1) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to one La(1), one Ti(3), and two equivalent Ti(2) atoms. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Ti(1) and two equivalent Ti(2) atoms. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one La(2), one Ti(1), and two equivalent Ti(2) atoms. In the sixth O site, O(6) is bonded in a linear geometry to one Si(1) and one Si(2) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to one Ti(3) and two equivalent Ti(2) atoms. In the eighth O site, O(8) is bonded in a distorted trigonal planar geometry to one Ti(1), one Ti(2), and one Ti(3) atom. In the ninth O site, O(9) is bonded in a 4-coordinate geometry to one La(1), one La(2), one Ti(4), and one Si(2) atom. In the tenth O site, O(10) is bonded in a bent 150 degrees geometry to one Ti(4) and one Si(1) atom. In the eleventh O site, O(11) is bonded in a distorted single-bond geometry to one La(1), one La(2), and one Si(2) atom.