Er5(ReO6)2 crystallizes in the monoclinic C2/m space group. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded to one O(3), two equivalent O(2), and four equivalent O(4) atoms to form distorted ErO7 pentagonal bipyramids that share corners with two equivalent Er(3)O6 octahedra,  an edgeedge with one Er(3)O6 octahedra, edges with two equivalent Re(1)O6 octahedra, and edges with two equivalent Er(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 56°. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one O(3), two equivalent O(4), and four equivalent O(2) atoms. In the third Er site, Er(3) is bonded to two equivalent O(1) and four equivalent O(4) atoms to form ErO6 octahedra that share corners with four equivalent Re(1)O6 octahedra, corners with four equivalent Er(1)O7 pentagonal bipyramids, and edges with two equivalent Er(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 38°. Re(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form ReO6 octahedra that share corners with two equivalent Er(3)O6 octahedra, edges with two equivalent Re(1)O6 octahedra, and edges with two equivalent Er(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 38°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Er(3) and two equivalent Re(1) atoms. In the second O site, O(2) is bonded to one Er(1), two equivalent Er(2), and one Re(1) atom to form distorted OEr3Re tetrahedra that share corners with four equivalent O(2)Er3Re tetrahedra, corners with six equivalent O(4)Er4 tetrahedra,  a cornercorner with one O(3)Er2Re2 trigonal pyramid,  an edgeedge with one O(4)Er4 tetrahedra, edges with two equivalent O(2)Er3Re tetrahedra, and edges with two equivalent O(3)Er2Re2 trigonal pyramids. In the third O site, O(3) is bonded to one Er(1), one Er(2), and two equivalent Re(1) atoms to form distorted OEr2Re2 trigonal pyramids that share corners with two equivalent O(2)Er3Re tetrahedra, corners with six equivalent O(4)Er4 tetrahedra, edges with four equivalent O(2)Er3Re tetrahedra, and  an edgeedge with one O(3)Er2Re2 trigonal pyramid. In the fourth O site, O(4) is bonded to one Er(2), one Er(3), and two equivalent Er(1) atoms to form OEr4 tetrahedra that share corners with four equivalent O(4)Er4 tetrahedra, corners with six equivalent O(2)Er3Re tetrahedra, corners with three equivalent O(3)Er2Re2 trigonal pyramids,  an edgeedge with one O(2)Er3Re tetrahedra, and edges with three equivalent O(4)Er4 tetrahedra.
Er5(ReO6)2 crystallizes in the monoclinic C2/m space group. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded to one O(3), two equivalent O(2), and four equivalent O(4) atoms to form distorted ErO7 pentagonal bipyramids that share corners with two equivalent Er(3)O6 octahedra,  an edgeedge with one Er(3)O6 octahedra, edges with two equivalent Re(1)O6 octahedra, and edges with two equivalent Er(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 56°. The Er(1)-O(3) bond length is 2.36 Å. Both Er(1)-O(2) bond lengths are 2.38 Å. There are two shorter (2.24 Å) and two longer (2.29 Å) Er(1)-O(4) bond lengths. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one O(3), two equivalent O(4), and four equivalent O(2) atoms. The Er(2)-O(3) bond length is 2.28 Å. Both Er(2)-O(4) bond lengths are 2.25 Å. There are two shorter (2.24 Å) and two longer (2.46 Å) Er(2)-O(2) bond lengths. In the third Er site, Er(3) is bonded to two equivalent O(1) and four equivalent O(4) atoms to form ErO6 octahedra that share corners with four equivalent Re(1)O6 octahedra, corners with four equivalent Er(1)O7 pentagonal bipyramids, and edges with two equivalent Er(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 38°. Both Er(3)-O(1) bond lengths are 2.16 Å. All Er(3)-O(4) bond lengths are 2.28 Å. Re(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form ReO6 octahedra that share corners with two equivalent Er(3)O6 octahedra, edges with two equivalent Re(1)O6 octahedra, and edges with two equivalent Er(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 38°. Both Re(1)-O(1) bond lengths are 1.96 Å. Both Re(1)-O(2) bond lengths are 1.95 Å. Both Re(1)-O(3) bond lengths are 2.08 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Er(3) and two equivalent Re(1) atoms. In the second O site, O(2) is bonded to one Er(1), two equivalent Er(2), and one Re(1) atom to form distorted OEr3Re tetrahedra that share corners with four equivalent O(2)Er3Re tetrahedra, corners with six equivalent O(4)Er4 tetrahedra,  a cornercorner with one O(3)Er2Re2 trigonal pyramid,  an edgeedge with one O(4)Er4 tetrahedra, edges with two equivalent O(2)Er3Re tetrahedra, and edges with two equivalent O(3)Er2Re2 trigonal pyramids. In the third O site, O(3) is bonded to one Er(1), one Er(2), and two equivalent Re(1) atoms to form distorted OEr2Re2 trigonal pyramids that share corners with two equivalent O(2)Er3Re tetrahedra, corners with six equivalent O(4)Er4 tetrahedra, edges with four equivalent O(2)Er3Re tetrahedra, and  an edgeedge with one O(3)Er2Re2 trigonal pyramid. In the fourth O site, O(4) is bonded to one Er(2), one Er(3), and two equivalent Er(1) atoms to form OEr4 tetrahedra that share corners with four equivalent O(4)Er4 tetrahedra, corners with six equivalent O(2)Er3Re tetrahedra, corners with three equivalent O(3)Er2Re2 trigonal pyramids,  an edgeedge with one O(2)Er3Re tetrahedra, and edges with three equivalent O(4)Er4 tetrahedra.