Er2TiO5 crystallizes in the monoclinic Cc space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(4), one O(5), and two equivalent O(2) atoms. In the second Er site, Er(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(4), and two equivalent O(5) atoms. Ti(1) is bonded to one O(1), one O(2), one O(3), one O(5), and two equivalent O(4) atoms to form corner-sharing TiO6 octahedra. The corner-sharing octahedral tilt angles are 51°. There are five inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Er(1), one Er(2), and one Ti(1) atom. In the second O site, O(2) is bonded to one Er(2), two equivalent Er(1), and one Ti(1) atom to form distorted OEr3Ti tetrahedra that share corners with two equivalent O(2)Er3Ti tetrahedra,  a cornercorner with one O(4)Er2Ti2 trigonal pyramid, and edges with two equivalent O(4)Er2Ti2 trigonal pyramids. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Er(1), one Er(2), and one Ti(1) atom. In the fourth O site, O(4) is bonded to one Er(1), one Er(2), and two equivalent Ti(1) atoms to form distorted OEr2Ti2 trigonal pyramids that share  a cornercorner with one O(2)Er3Ti tetrahedra, corners with two equivalent O(4)Er2Ti2 trigonal pyramids, and edges with two equivalent O(2)Er3Ti tetrahedra. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Er(1), two equivalent Er(2), and one Ti(1) atom.
Er2TiO5 crystallizes in the monoclinic Cc space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(4), one O(5), and two equivalent O(2) atoms. The Er(1)-O(1) bond length is 2.18 Å. The Er(1)-O(3) bond length is 2.17 Å. The Er(1)-O(4) bond length is 2.34 Å. The Er(1)-O(5) bond length is 2.52 Å. There is one shorter (2.22 Å) and one longer (2.23 Å) Er(1)-O(2) bond length. In the second Er site, Er(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(4), and two equivalent O(5) atoms. The Er(2)-O(1) bond length is 2.16 Å. The Er(2)-O(2) bond length is 2.52 Å. The Er(2)-O(3) bond length is 2.17 Å. The Er(2)-O(4) bond length is 2.33 Å. There is one shorter (2.22 Å) and one longer (2.23 Å) Er(2)-O(5) bond length. Ti(1) is bonded to one O(1), one O(2), one O(3), one O(5), and two equivalent O(4) atoms to form corner-sharing TiO6 octahedra. The corner-sharing octahedral tilt angles are 51°. The Ti(1)-O(1) bond length is 1.98 Å. The Ti(1)-O(2) bond length is 1.96 Å. The Ti(1)-O(3) bond length is 1.98 Å. The Ti(1)-O(5) bond length is 1.96 Å. There is one shorter (1.99 Å) and one longer (2.18 Å) Ti(1)-O(4) bond length. There are five inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Er(1), one Er(2), and one Ti(1) atom. In the second O site, O(2) is bonded to one Er(2), two equivalent Er(1), and one Ti(1) atom to form distorted OEr3Ti tetrahedra that share corners with two equivalent O(2)Er3Ti tetrahedra,  a cornercorner with one O(4)Er2Ti2 trigonal pyramid, and edges with two equivalent O(4)Er2Ti2 trigonal pyramids. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Er(1), one Er(2), and one Ti(1) atom. In the fourth O site, O(4) is bonded to one Er(1), one Er(2), and two equivalent Ti(1) atoms to form distorted OEr2Ti2 trigonal pyramids that share  a cornercorner with one O(2)Er3Ti tetrahedra, corners with two equivalent O(4)Er2Ti2 trigonal pyramids, and edges with two equivalent O(2)Er3Ti tetrahedra. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Er(1), two equivalent Er(2), and one Ti(1) atom.