Er2SiO5 crystallizes in the monoclinic C2/c space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one O(1), one O(2), two equivalent O(4), and two equivalent O(5) atoms to form distorted ErO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra and edges with two equivalent Er(1)O6 octahedra. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(5) atoms. 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 four equivalent Er(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-69°. There are five inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Er(1), one Er(2), and one Si(1) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Er(1), two equivalent Er(2), and one Si(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to two equivalent Er(2) and one Si(1) atom. In the fourth O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Er(1) and one Si(1) atom. In the fifth O site, O(5) is bonded to two equivalent Er(1) and two equivalent Er(2) atoms to form edge-sharing OEr4 tetrahedra.
Er2SiO5 crystallizes in the monoclinic C2/c space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one O(1), one O(2), two equivalent O(4), and two equivalent O(5) atoms to form distorted ErO6 octahedra that share corners with four equivalent Si(1)O4 tetrahedra and edges with two equivalent Er(1)O6 octahedra. The Er(1)-O(1) bond length is 2.25 Å. The Er(1)-O(2) bond length is 2.25 Å. Both Er(1)-O(4) bond lengths are 2.25 Å. There is one shorter (2.17 Å) and one longer (2.27 Å) Er(1)-O(5) bond length. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(5) atoms. The Er(2)-O(1) bond length is 2.29 Å. There is one shorter (2.35 Å) and one longer (2.60 Å) Er(2)-O(2) bond length. There is one shorter (2.27 Å) and one longer (2.30 Å) Er(2)-O(3) bond length. There is one shorter (2.17 Å) and one longer (2.35 Å) Er(2)-O(5) 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 four equivalent Er(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-69°. The Si(1)-O(1) bond length is 1.62 Å. The Si(1)-O(2) bond length is 1.64 Å. The Si(1)-O(3) bond length is 1.63 Å. The Si(1)-O(4) bond length is 1.63 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Er(1), one Er(2), and one Si(1) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Er(1), two equivalent Er(2), and one Si(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to two equivalent Er(2) and one Si(1) atom. In the fourth O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Er(1) and one Si(1) atom. In the fifth O site, O(5) is bonded to two equivalent Er(1) and two equivalent Er(2) atoms to form edge-sharing OEr4 tetrahedra.