ErAlGeO5 crystallizes in the orthorhombic Pbam space group. Er(1) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(3), and four equivalent O(1) atoms. Al(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form AlO6 octahedra that share corners with four equivalent Ge(1)O5 square pyramids and edges with two equivalent Al(1)O6 octahedra. Ge(1) is bonded to one O(4), two equivalent O(1), and two equivalent O(3) atoms to form distorted GeO5 square pyramids that share corners with four equivalent Al(1)O6 octahedra and  an edgeedge with one Ge(1)O5 square pyramid. The corner-sharing octahedral tilt angles range from 49-52°. There are four inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Er(1), one Al(1), and one Ge(1) atom to form distorted OEr2AlGe tetrahedra that share corners with four equivalent O(3)Er2Ge2 tetrahedra, corners with six equivalent O(1)Er2AlGe tetrahedra, corners with two equivalent O(2)Er2Al2 trigonal pyramids,  an edgeedge with one O(1)Er2AlGe tetrahedra,  an edgeedge with one O(3)Er2Ge2 tetrahedra, and edges with two equivalent O(2)Er2Al2 trigonal pyramids. In the second O site, O(2) is bonded to two equivalent Er(1) and two equivalent Al(1) atoms to form distorted OEr2Al2 trigonal pyramids that share corners with four equivalent O(1)Er2AlGe tetrahedra, corners with four equivalent O(3)Er2Ge2 tetrahedra, corners with two equivalent O(2)Er2Al2 trigonal pyramids, edges with four equivalent O(1)Er2AlGe tetrahedra, and  an edgeedge with one O(2)Er2Al2 trigonal pyramid. In the third O site, O(3) is bonded to two equivalent Er(1) and two equivalent Ge(1) atoms to form distorted OEr2Ge2 tetrahedra that share corners with eight equivalent O(1)Er2AlGe tetrahedra, corners with four equivalent O(2)Er2Al2 trigonal pyramids, edges with two equivalent O(1)Er2AlGe tetrahedra, and edges with two equivalent O(3)Er2Ge2 tetrahedra. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to two equivalent Al(1) and one Ge(1) atom.
ErAlGeO5 crystallizes in the orthorhombic Pbam space group. Er(1) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(3), and four equivalent O(1) atoms. There is one shorter (2.28 Å) and one longer (2.39 Å) Er(1)-O(2) bond length. Both Er(1)-O(3) bond lengths are 2.40 Å. There are two shorter (2.33 Å) and two longer (2.50 Å) Er(1)-O(1) bond lengths. Al(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form AlO6 octahedra that share corners with four equivalent Ge(1)O5 square pyramids and edges with two equivalent Al(1)O6 octahedra. Both Al(1)-O(1) bond lengths are 1.95 Å. Both Al(1)-O(2) bond lengths are 1.89 Å. Both Al(1)-O(4) bond lengths are 1.90 Å. Ge(1) is bonded to one O(4), two equivalent O(1), and two equivalent O(3) atoms to form distorted GeO5 square pyramids that share corners with four equivalent Al(1)O6 octahedra and  an edgeedge with one Ge(1)O5 square pyramid. The corner-sharing octahedral tilt angles range from 49-52°. The Ge(1)-O(4) bond length is 1.77 Å. Both Ge(1)-O(1) bond lengths are 1.85 Å. Both Ge(1)-O(3) bond lengths are 1.93 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Er(1), one Al(1), and one Ge(1) atom to form distorted OEr2AlGe tetrahedra that share corners with four equivalent O(3)Er2Ge2 tetrahedra, corners with six equivalent O(1)Er2AlGe tetrahedra, corners with two equivalent O(2)Er2Al2 trigonal pyramids,  an edgeedge with one O(1)Er2AlGe tetrahedra,  an edgeedge with one O(3)Er2Ge2 tetrahedra, and edges with two equivalent O(2)Er2Al2 trigonal pyramids. In the second O site, O(2) is bonded to two equivalent Er(1) and two equivalent Al(1) atoms to form distorted OEr2Al2 trigonal pyramids that share corners with four equivalent O(1)Er2AlGe tetrahedra, corners with four equivalent O(3)Er2Ge2 tetrahedra, corners with two equivalent O(2)Er2Al2 trigonal pyramids, edges with four equivalent O(1)Er2AlGe tetrahedra, and  an edgeedge with one O(2)Er2Al2 trigonal pyramid. In the third O site, O(3) is bonded to two equivalent Er(1) and two equivalent Ge(1) atoms to form distorted OEr2Ge2 tetrahedra that share corners with eight equivalent O(1)Er2AlGe tetrahedra, corners with four equivalent O(2)Er2Al2 trigonal pyramids, edges with two equivalent O(1)Er2AlGe tetrahedra, and edges with two equivalent O(3)Er2Ge2 tetrahedra. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to two equivalent Al(1) and one Ge(1) atom.