Er4(GeO5)3 crystallizes in the orthorhombic Cmcm 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(2), one O(3), one O(4), one O(6), two equivalent O(1), and two equivalent O(5) atoms. In the second Er site, Er(2) is bonded to one O(2), one O(3), one O(7), and four equivalent O(5) atoms to form ErO7 pentagonal bipyramids that share  a cornercorner with one Er(2)O7 pentagonal bipyramid, corners with two equivalent Ge(2)O4 tetrahedra, and edges with two equivalent Er(2)O7 pentagonal bipyramids. There are two inequivalent Ge sites. In the first Ge site, Ge(1) is bonded to two equivalent O(4) and four equivalent O(1) atoms to form GeO6 octahedra that share corners with two equivalent Ge(1)O6 octahedra and corners with four equivalent Ge(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 45°. In the second Ge site, Ge(2) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form GeO4 tetrahedra that share corners with two equivalent Ge(1)O6 octahedra and corners with two equivalent Er(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 60°. There are seven inequivalent O sites. In the first O site, O(2) is bonded in a distorted trigonal planar geometry to one Er(1), one Er(2), and one Ge(2) atom. In the second O site, O(3) is bonded in a trigonal planar geometry to one Er(1), one Er(2), and one Ge(2) atom. In the third O site, O(4) is bonded to two equivalent Er(1) and two equivalent Ge(1) atoms to form distorted corner-sharing OEr2Ge2 tetrahedra. In the fourth O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to one Er(1) and two equivalent Er(2) atoms. In the fifth O site, O(6) is bonded in a water-like geometry to two equivalent Er(1) atoms. In the sixth O site, O(7) is bonded in a linear geometry to two equivalent Er(2) atoms. In the seventh O site, O(1) is bonded in a trigonal planar geometry to one Er(1), one Ge(1), and one Ge(2) atom.
Er4(GeO5)3 crystallizes in the orthorhombic Cmcm 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(2), one O(3), one O(4), one O(6), two equivalent O(1), and two equivalent O(5) atoms. The Er(1)-O(2) bond length is 2.31 Å. The Er(1)-O(3) bond length is 2.32 Å. The Er(1)-O(4) bond length is 2.50 Å. The Er(1)-O(6) bond length is 2.58 Å. Both Er(1)-O(1) bond lengths are 2.36 Å. Both Er(1)-O(5) bond lengths are 2.27 Å. In the second Er site, Er(2) is bonded to one O(2), one O(3), one O(7), and four equivalent O(5) atoms to form ErO7 pentagonal bipyramids that share  a cornercorner with one Er(2)O7 pentagonal bipyramid, corners with two equivalent Ge(2)O4 tetrahedra, and edges with two equivalent Er(2)O7 pentagonal bipyramids. The Er(2)-O(2) bond length is 2.37 Å. The Er(2)-O(3) bond length is 2.23 Å. The Er(2)-O(7) bond length is 2.25 Å. There are two shorter (2.30 Å) and two longer (2.32 Å) Er(2)-O(5) bond lengths. There are two inequivalent Ge sites. In the first Ge site, Ge(1) is bonded to two equivalent O(4) and four equivalent O(1) atoms to form GeO6 octahedra that share corners with two equivalent Ge(1)O6 octahedra and corners with four equivalent Ge(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 45°. Both Ge(1)-O(4) bond lengths are 1.92 Å. All Ge(1)-O(1) bond lengths are 1.92 Å. In the second Ge site, Ge(2) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form GeO4 tetrahedra that share corners with two equivalent Ge(1)O6 octahedra and corners with two equivalent Er(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 60°. The Ge(2)-O(2) bond length is 1.76 Å. The Ge(2)-O(3) bond length is 1.75 Å. Both Ge(2)-O(1) bond lengths are 1.78 Å. There are seven inequivalent O sites. In the first O site, O(2) is bonded in a distorted trigonal planar geometry to one Er(1), one Er(2), and one Ge(2) atom. In the second O site, O(3) is bonded in a trigonal planar geometry to one Er(1), one Er(2), and one Ge(2) atom. In the third O site, O(4) is bonded to two equivalent Er(1) and two equivalent Ge(1) atoms to form distorted corner-sharing OEr2Ge2 tetrahedra. In the fourth O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to one Er(1) and two equivalent Er(2) atoms. In the fifth O site, O(6) is bonded in a water-like geometry to two equivalent Er(1) atoms. In the sixth O site, O(7) is bonded in a linear geometry to two equivalent Er(2) atoms. In the seventh O site, O(1) is bonded in a trigonal planar geometry to one Er(1), one Ge(1), and one Ge(2) atom.