Ba2Si3O8 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to one O(1), one O(3), one O(4), two equivalent O(7), and three equivalent O(2) atoms to form distorted BaO8 hexagonal bipyramids that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, corners with two equivalent Si(3)O4 tetrahedra, edges with four equivalent Ba(1)O8 hexagonal bipyramids, and edges with two equivalent Si(1)O4 tetrahedra. In the second Ba site, Ba(2) is bonded in a 8-coordinate geometry to one O(5), one O(7), one O(8), two equivalent O(4), and three equivalent O(6) atoms. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form SiO4 tetrahedra that share corners with two equivalent Ba(1)O8 hexagonal bipyramids,  a cornercorner with one Si(2)O4 tetrahedra, corners with two equivalent Si(1)O4 tetrahedra, and edges with two equivalent Ba(1)O8 hexagonal bipyramids. In the second Si site, Si(2) is bonded to one O(3), one O(4), one O(5), and one O(8) atom to form SiO4 tetrahedra that share corners with two equivalent Ba(1)O8 hexagonal bipyramids,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Si(3)O4 tetrahedra. In the third Si site, Si(3) is bonded to one O(5), one O(6), one O(7), and one O(8) atom to form SiO4 tetrahedra that share corners with two equivalent Ba(1)O8 hexagonal bipyramids and corners with two equivalent Si(2)O4 tetrahedra. There are eight inequivalent O sites. In the first O site, O(4) is bonded in a distorted single-bond geometry to one Ba(1), two equivalent Ba(2), and one Si(2) atom. In the second O site, O(5) is bonded in a 2-coordinate geometry to one Ba(2), one Si(2), and one Si(3) atom. In the third O site, O(6) is bonded in a distorted single-bond geometry to three equivalent Ba(2) and one Si(3) atom. In the fourth O site, O(7) is bonded in a distorted single-bond geometry to one Ba(2), two equivalent Ba(1), and one Si(3) atom. In the fifth O site, O(8) is bonded in a distorted bent 120 degrees geometry to one Ba(2), one Si(2), and one Si(3) atom. In the sixth O site, O(1) is bonded in a 2-coordinate geometry to one Ba(1) and two equivalent Si(1) atoms. In the seventh O site, O(2) is bonded in a distorted single-bond geometry to three equivalent Ba(1) and one Si(1) atom. In the eighth O site, O(3) is bonded in a distorted bent 150 degrees geometry to one Ba(1), one Si(1), and one Si(2) atom.
Ba2Si3O8 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to one O(1), one O(3), one O(4), two equivalent O(7), and three equivalent O(2) atoms to form distorted BaO8 hexagonal bipyramids that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, corners with two equivalent Si(3)O4 tetrahedra, edges with four equivalent Ba(1)O8 hexagonal bipyramids, and edges with two equivalent Si(1)O4 tetrahedra. The Ba(1)-O(1) bond length is 3.06 Å. The Ba(1)-O(3) bond length is 2.89 Å. The Ba(1)-O(4) bond length is 2.69 Å. There is one shorter (2.81 Å) and one longer (2.96 Å) Ba(1)-O(7) bond length. There are a spread of Ba(1)-O(2) bond distances ranging from 2.75-2.91 Å. In the second Ba site, Ba(2) is bonded in a 8-coordinate geometry to one O(5), one O(7), one O(8), two equivalent O(4), and three equivalent O(6) atoms. The Ba(2)-O(5) bond length is 2.80 Å. The Ba(2)-O(7) bond length is 2.75 Å. The Ba(2)-O(8) bond length is 3.04 Å. There is one shorter (2.82 Å) and one longer (3.07 Å) Ba(2)-O(4) bond length. There are a spread of Ba(2)-O(6) bond distances ranging from 2.67-2.83 Å. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form SiO4 tetrahedra that share corners with two equivalent Ba(1)O8 hexagonal bipyramids,  a cornercorner with one Si(2)O4 tetrahedra, corners with two equivalent Si(1)O4 tetrahedra, and edges with two equivalent Ba(1)O8 hexagonal bipyramids. The Si(1)-O(2) bond length is 1.58 Å. The Si(1)-O(3) bond length is 1.64 Å. Both Si(1)-O(1) bond lengths are 1.64 Å. In the second Si site, Si(2) is bonded to one O(3), one O(4), one O(5), and one O(8) atom to form SiO4 tetrahedra that share corners with two equivalent Ba(1)O8 hexagonal bipyramids,  a cornercorner with one Si(1)O4 tetrahedra, and corners with two equivalent Si(3)O4 tetrahedra. The Si(2)-O(3) bond length is 1.64 Å. The Si(2)-O(4) bond length is 1.58 Å. The Si(2)-O(5) bond length is 1.64 Å. The Si(2)-O(8) bond length is 1.64 Å. In the third Si site, Si(3) is bonded to one O(5), one O(6), one O(7), and one O(8) atom to form SiO4 tetrahedra that share corners with two equivalent Ba(1)O8 hexagonal bipyramids and corners with two equivalent Si(2)O4 tetrahedra. The Si(3)-O(5) bond length is 1.66 Å. The Si(3)-O(6) bond length is 1.60 Å. The Si(3)-O(7) bond length is 1.60 Å. The Si(3)-O(8) bond length is 1.66 Å. There are eight inequivalent O sites. In the first O site, O(4) is bonded in a distorted single-bond geometry to one Ba(1), two equivalent Ba(2), and one Si(2) atom. In the second O site, O(5) is bonded in a 2-coordinate geometry to one Ba(2), one Si(2), and one Si(3) atom. In the third O site, O(6) is bonded in a distorted single-bond geometry to three equivalent Ba(2) and one Si(3) atom. In the fourth O site, O(7) is bonded in a distorted single-bond geometry to one Ba(2), two equivalent Ba(1), and one Si(3) atom. In the fifth O site, O(8) is bonded in a distorted bent 120 degrees geometry to one Ba(2), one Si(2), and one Si(3) atom. In the sixth O site, O(1) is bonded in a 2-coordinate geometry to one Ba(1) and two equivalent Si(1) atoms. In the seventh O site, O(2) is bonded in a distorted single-bond geometry to three equivalent Ba(1) and one Si(1) atom. In the eighth O site, O(3) is bonded in a distorted bent 150 degrees geometry to one Ba(1), one Si(1), and one Si(2) atom.