Er2GeO5 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 8-coordinate geometry to one O(5), two equivalent O(1), two equivalent O(3), and three equivalent O(2) atoms. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one O(3), three equivalent O(4), and three equivalent O(5) atoms. Ge(1) is bonded in a tetrahedral geometry to one O(1), one O(2), one O(3), and one O(4) atom. There are five inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Er(1) and one Ge(1) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to three equivalent Er(1) and one Ge(1) atom. In the third O site, O(3) is bonded to one Er(2), two equivalent Er(1), and one Ge(1) atom to form distorted OEr3Ge tetrahedra that share  a cornercorner with one O(5)Er4 tetrahedra, corners with two equivalent O(3)Er3Ge tetrahedra, corners with two equivalent O(4)Er3Ge trigonal pyramids, edges with two equivalent O(5)Er4 tetrahedra, and  an edgeedge with one O(4)Er3Ge trigonal pyramid. In the fourth O site, O(4) is bonded to three equivalent Er(2) and one Ge(1) atom to form distorted OEr3Ge trigonal pyramids that share corners with two equivalent O(3)Er3Ge tetrahedra, corners with five equivalent O(5)Er4 tetrahedra, corners with four equivalent O(4)Er3Ge trigonal pyramids,  an edgeedge with one O(3)Er3Ge tetrahedra, edges with two equivalent O(5)Er4 tetrahedra, and  an edgeedge with one O(4)Er3Ge trigonal pyramid. In the fifth O site, O(5) is bonded to one Er(1) and three equivalent Er(2) atoms to form OEr4 tetrahedra that share  a cornercorner with one O(3)Er3Ge tetrahedra, corners with four equivalent O(5)Er4 tetrahedra, corners with five equivalent O(4)Er3Ge trigonal pyramids,  an edgeedge with one O(5)Er4 tetrahedra, edges with two equivalent O(3)Er3Ge tetrahedra, and edges with two equivalent O(4)Er3Ge trigonal pyramids.
Er2GeO5 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 8-coordinate geometry to one O(5), two equivalent O(1), two equivalent O(3), and three equivalent O(2) atoms. The Er(1)-O(5) bond length is 2.28 Å. There is one shorter (2.36 Å) and one longer (2.39 Å) Er(1)-O(1) bond length. There is one shorter (2.25 Å) and one longer (2.43 Å) Er(1)-O(3) bond length. There are a spread of Er(1)-O(2) bond distances ranging from 2.30-2.78 Å. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one O(3), three equivalent O(4), and three equivalent O(5) atoms. The Er(2)-O(3) bond length is 2.30 Å. There are a spread of Er(2)-O(4) bond distances ranging from 2.35-2.54 Å. There are a spread of Er(2)-O(5) bond distances ranging from 2.23-2.32 Å. Ge(1) is bonded in a tetrahedral geometry to one O(1), one O(2), one O(3), and one O(4) atom. The Ge(1)-O(1) bond length is 1.73 Å. The Ge(1)-O(2) bond length is 1.77 Å. The Ge(1)-O(3) bond length is 1.82 Å. The Ge(1)-O(4) bond length is 1.80 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Er(1) and one Ge(1) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to three equivalent Er(1) and one Ge(1) atom. In the third O site, O(3) is bonded to one Er(2), two equivalent Er(1), and one Ge(1) atom to form distorted OEr3Ge tetrahedra that share  a cornercorner with one O(5)Er4 tetrahedra, corners with two equivalent O(3)Er3Ge tetrahedra, corners with two equivalent O(4)Er3Ge trigonal pyramids, edges with two equivalent O(5)Er4 tetrahedra, and  an edgeedge with one O(4)Er3Ge trigonal pyramid. In the fourth O site, O(4) is bonded to three equivalent Er(2) and one Ge(1) atom to form distorted OEr3Ge trigonal pyramids that share corners with two equivalent O(3)Er3Ge tetrahedra, corners with five equivalent O(5)Er4 tetrahedra, corners with four equivalent O(4)Er3Ge trigonal pyramids,  an edgeedge with one O(3)Er3Ge tetrahedra, edges with two equivalent O(5)Er4 tetrahedra, and  an edgeedge with one O(4)Er3Ge trigonal pyramid. In the fifth O site, O(5) is bonded to one Er(1) and three equivalent Er(2) atoms to form OEr4 tetrahedra that share  a cornercorner with one O(3)Er3Ge tetrahedra, corners with four equivalent O(5)Er4 tetrahedra, corners with five equivalent O(4)Er3Ge trigonal pyramids,  an edgeedge with one O(5)Er4 tetrahedra, edges with two equivalent O(3)Er3Ge tetrahedra, and edges with two equivalent O(4)Er3Ge trigonal pyramids.