Er(Bi3O5)4 crystallizes in the cubic I23 space group. Er(1) is bonded to four equivalent O(3) atoms to form ErO4 tetrahedra that share corners with twelve equivalent Bi(1)O5 square pyramids. Bi(1) is bonded to one O(2), one O(3), and three equivalent O(1) atoms to form distorted BiO5 square pyramids that share corners with eight equivalent Bi(1)O5 square pyramids,  a cornercorner with one Er(1)O4 tetrahedra, and  an edgeedge with one Bi(1)O5 square pyramid. There are three inequivalent O sites. In the first O site, O(3) is bonded to one Er(1) and three equivalent Bi(1) atoms to form corner-sharing OErBi3 tetrahedra. In the second O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to three equivalent Bi(1) atoms. In the third O site, O(2) is bonded in a distorted trigonal planar geometry to three equivalent Bi(1) atoms.
Er(Bi3O5)4 crystallizes in the cubic I23 space group. Er(1) is bonded to four equivalent O(3) atoms to form ErO4 tetrahedra that share corners with twelve equivalent Bi(1)O5 square pyramids. All Er(1)-O(3) bond lengths are 2.15 Å. Bi(1) is bonded to one O(2), one O(3), and three equivalent O(1) atoms to form distorted BiO5 square pyramids that share corners with eight equivalent Bi(1)O5 square pyramids,  a cornercorner with one Er(1)O4 tetrahedra, and  an edgeedge with one Bi(1)O5 square pyramid. The Bi(1)-O(2) bond length is 2.27 Å. The Bi(1)-O(3) bond length is 2.42 Å. There are a spread of Bi(1)-O(1) bond distances ranging from 2.09-2.55 Å. There are three inequivalent O sites. In the first O site, O(3) is bonded to one Er(1) and three equivalent Bi(1) atoms to form corner-sharing OErBi3 tetrahedra. In the second O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to three equivalent Bi(1) atoms. In the third O site, O(2) is bonded in a distorted trigonal planar geometry to three equivalent Bi(1) atoms.