Y2Si2O7 crystallizes in the monoclinic P2_1/m space group. Y(1) is bonded to one O(3), one O(4), one O(5), two equivalent O(1), and two equivalent O(2) atoms to form distorted YO7 pentagonal bipyramids that share  a cornercorner with one Y(1)O7 pentagonal bipyramid, corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, edges with three equivalent Y(1)O7 pentagonal bipyramids,  an edgeedge with one Si(1)O4 tetrahedra, and  an edgeedge with one Si(2)O4 tetrahedra. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(3), one O(5), and two equivalent O(1) atoms to form SiO4 tetrahedra that share corners with four equivalent Y(1)O7 pentagonal bipyramids,  a cornercorner with one Si(2)O4 tetrahedra, and edges with two equivalent Y(1)O7 pentagonal bipyramids. In the second Si site, Si(2) is bonded to one O(3), one O(4), and two equivalent O(2) atoms to form SiO4 tetrahedra that share corners with four equivalent Y(1)O7 pentagonal bipyramids,  a cornercorner with one Si(1)O4 tetrahedra, and edges with two equivalent Y(1)O7 pentagonal bipyramids. There are five inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Y(1) and one Si(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Y(1) and one Si(2) atom. In the third O site, O(3) is bonded in a distorted bent 150 degrees geometry to two equivalent Y(1), one Si(1), and one Si(2) atom. In the fourth O site, O(4) is bonded in a trigonal planar geometry to two equivalent Y(1) and one Si(2) atom. In the fifth O site, O(5) is bonded in a trigonal planar geometry to two equivalent Y(1) and one Si(1) atom.
Y2Si2O7 crystallizes in the monoclinic P2_1/m space group. Y(1) is bonded to one O(3), one O(4), one O(5), two equivalent O(1), and two equivalent O(2) atoms to form distorted YO7 pentagonal bipyramids that share  a cornercorner with one Y(1)O7 pentagonal bipyramid, corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, edges with three equivalent Y(1)O7 pentagonal bipyramids,  an edgeedge with one Si(1)O4 tetrahedra, and  an edgeedge with one Si(2)O4 tetrahedra. The Y(1)-O(3) bond length is 2.50 Å. The Y(1)-O(4) bond length is 2.35 Å. The Y(1)-O(5) bond length is 2.34 Å. There is one shorter (2.23 Å) and one longer (2.41 Å) Y(1)-O(1) bond length. There is one shorter (2.27 Å) and one longer (2.49 Å) Y(1)-O(2) bond length. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(3), one O(5), and two equivalent O(1) atoms to form SiO4 tetrahedra that share corners with four equivalent Y(1)O7 pentagonal bipyramids,  a cornercorner with one Si(2)O4 tetrahedra, and edges with two equivalent Y(1)O7 pentagonal bipyramids. The Si(1)-O(3) bond length is 1.70 Å. The Si(1)-O(5) bond length is 1.62 Å. Both Si(1)-O(1) bond lengths are 1.63 Å. In the second Si site, Si(2) is bonded to one O(3), one O(4), and two equivalent O(2) atoms to form SiO4 tetrahedra that share corners with four equivalent Y(1)O7 pentagonal bipyramids,  a cornercorner with one Si(1)O4 tetrahedra, and edges with two equivalent Y(1)O7 pentagonal bipyramids. The Si(2)-O(3) bond length is 1.69 Å. The Si(2)-O(4) bond length is 1.63 Å. Both Si(2)-O(2) bond lengths are 1.63 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Y(1) and one Si(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Y(1) and one Si(2) atom. In the third O site, O(3) is bonded in a distorted bent 150 degrees geometry to two equivalent Y(1), one Si(1), and one Si(2) atom. In the fourth O site, O(4) is bonded in a trigonal planar geometry to two equivalent Y(1) and one Si(2) atom. In the fifth O site, O(5) is bonded in a trigonal planar geometry to two equivalent Y(1) and one Si(1) atom.