Cs2B4SiO9 crystallizes in the monoclinic C2 space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 9-coordinate geometry to one O(5), two equivalent O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms. In the second Cs site, Cs(2) is bonded in a 10-coordinate geometry to two equivalent O(1), two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. There are two inequivalent B sites. In the first B site, B(1) is bonded to one O(1), one O(3), one O(4), and one O(5) atom to form BO4 tetrahedra that share  a cornercorner with one B(1)O4 tetrahedra and  a cornercorner with one Si(1)O4 tetrahedra. In the second B site, B(2) is bonded in a trigonal planar geometry to one O(2), one O(3), and one O(4) atom. Si(1) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form SiO4 tetrahedra that share corners with two equivalent B(1)O4 tetrahedra. There are five inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 120 degrees geometry to one Cs(1), one Cs(2), one B(1), and one Si(1) atom. In the second O site, O(2) is bonded in a distorted bent 120 degrees geometry to one Cs(1), one Cs(2), one B(2), and one Si(1) atom. In the third O site, O(3) is bonded in a bent 120 degrees geometry to one Cs(1), one Cs(2), one B(1), and one B(2) atom. In the fourth O site, O(4) is bonded in a bent 120 degrees geometry to one Cs(1), one Cs(2), one B(1), and one B(2) atom. In the fifth O site, O(5) is bonded in a water-like geometry to one Cs(1), two equivalent Cs(2), and two equivalent B(1) atoms.
Cs2B4SiO9 crystallizes in the monoclinic C2 space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 9-coordinate geometry to one O(5), two equivalent O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms. The Cs(1)-O(5) bond length is 3.32 Å. Both Cs(1)-O(1) bond lengths are 3.24 Å. Both Cs(1)-O(2) bond lengths are 3.19 Å. Both Cs(1)-O(3) bond lengths are 3.30 Å. Both Cs(1)-O(4) bond lengths are 3.18 Å. In the second Cs site, Cs(2) is bonded in a 10-coordinate geometry to two equivalent O(1), two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. Both Cs(2)-O(1) bond lengths are 3.65 Å. Both Cs(2)-O(2) bond lengths are 3.51 Å. Both Cs(2)-O(3) bond lengths are 3.13 Å. Both Cs(2)-O(4) bond lengths are 3.40 Å. Both Cs(2)-O(5) bond lengths are 3.56 Å. There are two inequivalent B sites. In the first B site, B(1) is bonded to one O(1), one O(3), one O(4), and one O(5) atom to form BO4 tetrahedra that share  a cornercorner with one B(1)O4 tetrahedra and  a cornercorner with one Si(1)O4 tetrahedra. The B(1)-O(1) bond length is 1.46 Å. The B(1)-O(3) bond length is 1.51 Å. The B(1)-O(4) bond length is 1.53 Å. The B(1)-O(5) bond length is 1.45 Å. In the second B site, B(2) is bonded in a trigonal planar geometry to one O(2), one O(3), and one O(4) atom. The B(2)-O(2) bond length is 1.39 Å. The B(2)-O(3) bond length is 1.37 Å. The B(2)-O(4) bond length is 1.37 Å. Si(1) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form SiO4 tetrahedra that share corners with two equivalent B(1)O4 tetrahedra. Both Si(1)-O(1) bond lengths are 1.63 Å. Both Si(1)-O(2) bond lengths are 1.66 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 120 degrees geometry to one Cs(1), one Cs(2), one B(1), and one Si(1) atom. In the second O site, O(2) is bonded in a distorted bent 120 degrees geometry to one Cs(1), one Cs(2), one B(2), and one Si(1) atom. In the third O site, O(3) is bonded in a bent 120 degrees geometry to one Cs(1), one Cs(2), one B(1), and one B(2) atom. In the fourth O site, O(4) is bonded in a bent 120 degrees geometry to one Cs(1), one Cs(2), one B(1), and one B(2) atom. In the fifth O site, O(5) is bonded in a water-like geometry to one Cs(1), two equivalent Cs(2), and two equivalent B(1) atoms.