Er2Si2O7 crystallizes in the monoclinic P2_1/c space group. Er(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted ErO6 octahedra that share corners with six equivalent Si(1)O4 tetrahedra and edges with three equivalent Er(1)O6 octahedra. Si(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SiO4 tetrahedra that share corners with six equivalent Er(1)O6 octahedra and  a cornercorner with one Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 42-61°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Er(1) and one Si(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Er(1) and one Si(1) atom. In the third O site, O(3) is bonded in a linear geometry to two equivalent Si(1) atoms. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to two equivalent Er(1) and one Si(1) atom.
Er2Si2O7 crystallizes in the monoclinic P2_1/c space group. Er(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted ErO6 octahedra that share corners with six equivalent Si(1)O4 tetrahedra and edges with three equivalent Er(1)O6 octahedra. There is one shorter (2.22 Å) and one longer (2.25 Å) Er(1)-O(1) bond length. There is one shorter (2.22 Å) and one longer (2.25 Å) Er(1)-O(2) bond length. There is one shorter (2.22 Å) and one longer (2.29 Å) Er(1)-O(4) bond length. Si(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SiO4 tetrahedra that share corners with six equivalent Er(1)O6 octahedra and  a cornercorner with one Si(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 42-61°. The Si(1)-O(1) bond length is 1.62 Å. The Si(1)-O(2) bond length is 1.62 Å. The Si(1)-O(3) bond length is 1.63 Å. The Si(1)-O(4) bond length is 1.64 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Er(1) and one Si(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Er(1) and one Si(1) atom. In the third O site, O(3) is bonded in a linear geometry to two equivalent Si(1) atoms. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to two equivalent Er(1) and one Si(1) atom.