Cs2Ti4O9 crystallizes in the monoclinic C2/m space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(6), and four equivalent O(5) atoms. In the second Cs site, Cs(2) is bonded in a 6-coordinate geometry to two equivalent O(3), two equivalent O(5), and two equivalent O(9) atoms. There are four inequivalent Ti sites. In the first Ti site, Ti(1) is bonded in a 6-coordinate geometry to one O(2), one O(4), one O(6), one O(8), and two equivalent O(1) atoms. In the second Ti site, Ti(2) is bonded to one O(2), one O(3), one O(4), and three equivalent O(7) atoms to form a mixture of distorted corner and edge-sharing TiO6 octahedra. The corner-sharing octahedral tilt angles are 29°. In the third Ti site, Ti(3) is bonded to one O(1), one O(3), one O(7), one O(9), and two equivalent O(4) atoms to form a mixture of distorted corner and edge-sharing TiO6 octahedra. The corner-sharing octahedral tilt angles are 30°. In the fourth Ti site, Ti(4) is bonded to one O(1), one O(5), one O(6), one O(9), and two equivalent O(8) atoms to form a mixture of distorted corner and edge-sharing TiO6 octahedra. The corner-sharing octahedral tilt angles are 31°. There are nine inequivalent O sites. In the first O site, O(5) is bonded in a single-bond geometry to two equivalent Cs(2), four equivalent Cs(1), and one Ti(4) atom. In the second O site, O(6) is bonded in a distorted linear geometry to two equivalent Cs(1), one Ti(1), and one Ti(4) atom. In the third O site, O(7) is bonded to one Ti(3) and three equivalent Ti(2) atoms to form a mixture of distorted corner and edge-sharing OTi4 trigonal pyramids. In the fourth O site, O(8) is bonded in a 3-coordinate geometry to one Ti(1) and two equivalent Ti(4) atoms. In the fifth O site, O(9) is bonded in a 4-coordinate geometry to two equivalent Cs(2), one Ti(3), and one Ti(4) atom. In the sixth O site, O(1) is bonded to one Ti(3), one Ti(4), and two equivalent Ti(1) atoms to form a mixture of distorted corner and edge-sharing OTi4 trigonal pyramids. In the seventh O site, O(2) is bonded in a 4-coordinate geometry to two equivalent Cs(1), one Ti(1), and one Ti(2) atom. In the eighth O site, O(3) is bonded in a distorted water-like geometry to two equivalent Cs(2), one Ti(2), and one Ti(3) atom. In the ninth O site, O(4) is bonded to one Ti(1), one Ti(2), and two equivalent Ti(3) atoms to form a mixture of distorted corner and edge-sharing OTi4 trigonal pyramids.
Cs2Ti4O9 crystallizes in the monoclinic C2/m space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(6), and four equivalent O(5) atoms. Both Cs(1)-O(2) bond lengths are 2.91 Å. Both Cs(1)-O(6) bond lengths are 3.06 Å. There are two shorter (3.09 Å) and two longer (3.57 Å) Cs(1)-O(5) bond lengths. In the second Cs site, Cs(2) is bonded in a 6-coordinate geometry to two equivalent O(3), two equivalent O(5), and two equivalent O(9) atoms. Both Cs(2)-O(3) bond lengths are 3.10 Å. Both Cs(2)-O(5) bond lengths are 3.18 Å. Both Cs(2)-O(9) bond lengths are 2.97 Å. There are four inequivalent Ti sites. In the first Ti site, Ti(1) is bonded in a 6-coordinate geometry to one O(2), one O(4), one O(6), one O(8), and two equivalent O(1) atoms. The Ti(1)-O(2) bond length is 2.01 Å. The Ti(1)-O(4) bond length is 2.29 Å. The Ti(1)-O(6) bond length is 1.76 Å. The Ti(1)-O(8) bond length is 1.99 Å. Both Ti(1)-O(1) bond lengths are 1.97 Å. In the second Ti site, Ti(2) is bonded to one O(2), one O(3), one O(4), and three equivalent O(7) atoms to form a mixture of distorted corner and edge-sharing TiO6 octahedra. The corner-sharing octahedral tilt angles are 29°. The Ti(2)-O(2) bond length is 1.79 Å. The Ti(2)-O(3) bond length is 1.87 Å. The Ti(2)-O(4) bond length is 2.12 Å. There are two shorter (1.98 Å) and one longer (2.17 Å) Ti(2)-O(7) bond length. In the third Ti site, Ti(3) is bonded to one O(1), one O(3), one O(7), one O(9), and two equivalent O(4) atoms to form a mixture of distorted corner and edge-sharing TiO6 octahedra. The corner-sharing octahedral tilt angles are 30°. The Ti(3)-O(1) bond length is 2.14 Å. The Ti(3)-O(3) bond length is 1.89 Å. The Ti(3)-O(7) bond length is 2.21 Å. The Ti(3)-O(9) bond length is 1.78 Å. Both Ti(3)-O(4) bond lengths are 1.98 Å. In the fourth Ti site, Ti(4) is bonded to one O(1), one O(5), one O(6), one O(9), and two equivalent O(8) atoms to form a mixture of distorted corner and edge-sharing TiO6 octahedra. The corner-sharing octahedral tilt angles are 31°. The Ti(4)-O(1) bond length is 2.33 Å. The Ti(4)-O(5) bond length is 1.73 Å. The Ti(4)-O(6) bond length is 2.00 Å. The Ti(4)-O(9) bond length is 2.06 Å. Both Ti(4)-O(8) bond lengths are 1.99 Å. There are nine inequivalent O sites. In the first O site, O(5) is bonded in a single-bond geometry to two equivalent Cs(2), four equivalent Cs(1), and one Ti(4) atom. In the second O site, O(6) is bonded in a distorted linear geometry to two equivalent Cs(1), one Ti(1), and one Ti(4) atom. In the third O site, O(7) is bonded to one Ti(3) and three equivalent Ti(2) atoms to form a mixture of distorted corner and edge-sharing OTi4 trigonal pyramids. In the fourth O site, O(8) is bonded in a 3-coordinate geometry to one Ti(1) and two equivalent Ti(4) atoms. In the fifth O site, O(9) is bonded in a 4-coordinate geometry to two equivalent Cs(2), one Ti(3), and one Ti(4) atom. In the sixth O site, O(1) is bonded to one Ti(3), one Ti(4), and two equivalent Ti(1) atoms to form a mixture of distorted corner and edge-sharing OTi4 trigonal pyramids. In the seventh O site, O(2) is bonded in a 4-coordinate geometry to two equivalent Cs(1), one Ti(1), and one Ti(2) atom. In the eighth O site, O(3) is bonded in a distorted water-like geometry to two equivalent Cs(2), one Ti(2), and one Ti(3) atom. In the ninth O site, O(4) is bonded to one Ti(1), one Ti(2), and two equivalent Ti(3) atoms to form a mixture of distorted corner and edge-sharing OTi4 trigonal pyramids.