Be2Er2SiO7 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. Er(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 Er(1) atoms to form a mixture of distorted edge and corner-sharing OEr2Be2 tetrahedra. In the second O site, O(2) is bonded to one Be(1) and three equivalent Er(1) atoms to form distorted OEr3Be tetrahedra that share corners with two equivalent O(1)Er2Be2 tetrahedra, corners with four equivalent O(2)Er3Be tetrahedra,  an edgeedge with one O(1)Er2Be2 tetrahedra, and  an edgeedge with one O(2)Er3Be tetrahedra. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Be(1), two equivalent Er(1), and one Si(1) atom.
Be2Er2SiO7 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 1.56 Å. The Be(1)-O(2) bond length is 1.59 Å. Both Be(1)-O(3) bond lengths are 1.69 Å. Er(1) is bonded in a 8-coordinate geometry to one O(1), three equivalent O(2), and four equivalent O(3) atoms. The Er(1)-O(1) bond length is 2.24 Å. There is one shorter (2.31 Å) and two longer (2.40 Å) Er(1)-O(2) bond lengths. There are two shorter (2.34 Å) and two longer (2.61 Å) Er(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 1.64 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Be(1) and two equivalent Er(1) atoms to form a mixture of distorted edge and corner-sharing OEr2Be2 tetrahedra. In the second O site, O(2) is bonded to one Be(1) and three equivalent Er(1) atoms to form distorted OEr3Be tetrahedra that share corners with two equivalent O(1)Er2Be2 tetrahedra, corners with four equivalent O(2)Er3Be tetrahedra,  an edgeedge with one O(1)Er2Be2 tetrahedra, and  an edgeedge with one O(2)Er3Be tetrahedra. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Be(1), two equivalent Er(1), and one Si(1) atom.