Be2Dy2SiO7 crystallizes in the tetragonal P-42_1m space group. Be(1) is bonded to one O(1), one O(2), and two equivalent O(3) atoms to form BeO4 tetrahedra that share  a cornercorner with one Be(1)O4 tetrahedra and corners with two equivalent Si(1)O4 tetrahedra. Dy(1) is bonded in a 8-coordinate geometry to one O(1), three equivalent O(2), and four equivalent O(3) atoms. Si(1) is bonded to four equivalent O(3) atoms to form SiO4 tetrahedra that share corners with four equivalent Be(1)O4 tetrahedra. There are three inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Be(1) and two equivalent Dy(1) atoms to form a mixture of distorted corner and edge-sharing ODy2Be2 tetrahedra. In the second O site, O(2) is bonded to one Be(1) and three equivalent Dy(1) atoms to form distorted ODy3Be tetrahedra that share corners with two equivalent O(1)Dy2Be2 tetrahedra, corners with four equivalent O(2)Dy3Be tetrahedra,  an edgeedge with one O(1)Dy2Be2 tetrahedra, and  an edgeedge with one O(2)Dy3Be tetrahedra. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Be(1), two equivalent Dy(1), and one Si(1) atom.
Be2Dy2SiO7 crystallizes in the tetragonal P-42_1m space group. Be(1) is bonded to one O(1), one O(2), and two equivalent O(3) atoms to form BeO4 tetrahedra that share  a cornercorner with one Be(1)O4 tetrahedra and corners with two equivalent Si(1)O4 tetrahedra. The Be(1)-O(1) bond length is 1.57 Å. The Be(1)-O(2) bond length is 1.59 Å. Both Be(1)-O(3) bond lengths are 1.71 Å. Dy(1) is bonded in a 8-coordinate geometry to one O(1), three equivalent O(2), and four equivalent O(3) atoms. The Dy(1)-O(1) bond length is 2.27 Å. There is one shorter (2.34 Å) and two longer (2.40 Å) Dy(1)-O(2) bond lengths. There are two shorter (2.36 Å) and two longer (2.65 Å) Dy(1)-O(3) bond lengths. Si(1) is bonded to four equivalent O(3) atoms to form SiO4 tetrahedra that share corners with four equivalent Be(1)O4 tetrahedra. All Si(1)-O(3) bond lengths are 1.65 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Be(1) and two equivalent Dy(1) atoms to form a mixture of distorted corner and edge-sharing ODy2Be2 tetrahedra. In the second O site, O(2) is bonded to one Be(1) and three equivalent Dy(1) atoms to form distorted ODy3Be tetrahedra that share corners with two equivalent O(1)Dy2Be2 tetrahedra, corners with four equivalent O(2)Dy3Be tetrahedra,  an edgeedge with one O(1)Dy2Be2 tetrahedra, and  an edgeedge with one O(2)Dy3Be tetrahedra. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Be(1), two equivalent Dy(1), and one Si(1) atom.