Er3TaO7 crystallizes in the orthorhombic C222_1 space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one O(1), one O(4), one O(5), two equivalent O(2), and two equivalent O(3) atoms to form distorted ErO7 pentagonal bipyramids that share corners with two equivalent Ta(1)O6 octahedra,  a cornercorner with one Er(1)O7 pentagonal bipyramid, edges with two equivalent Ta(1)O6 octahedra, and edges with three equivalent Er(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 39-50°. In the second Er site, Er(2) is bonded in a 6-coordinate geometry to two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. Ta(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with four equivalent Er(1)O7 pentagonal bipyramids, and edges with four equivalent Er(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 44°. There are five inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Er(1) and two equivalent Ta(1) atoms to form distorted OEr2Ta2 trigonal pyramids that share corners with two equivalent O(5)Er4 tetrahedra, corners with four equivalent O(3)Er3Ta tetrahedra, corners with two equivalent O(1)Er2Ta2 trigonal pyramids,  an edgeedge with one O(4)Er4 tetrahedra, and edges with two equivalent O(3)Er3Ta tetrahedra. In the second O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Er(1) and one Ta(1) atom. In the third O site, O(3) is bonded to one Er(2), two equivalent Er(1), and one Ta(1) atom to form distorted OEr3Ta tetrahedra that share corners with two equivalent O(3)Er3Ta tetrahedra, corners with two equivalent O(4)Er4 tetrahedra, corners with two equivalent O(5)Er4 tetrahedra, corners with two equivalent O(1)Er2Ta2 trigonal pyramids,  an edgeedge with one O(3)Er3Ta tetrahedra,  an edgeedge with one O(4)Er4 tetrahedra,  an edgeedge with one O(5)Er4 tetrahedra, and  an edgeedge with one O(1)Er2Ta2 trigonal pyramid. In the fourth O site, O(4) is bonded to two equivalent Er(1) and two equivalent Er(2) atoms to form OEr4 tetrahedra that share corners with two equivalent O(4)Er4 tetrahedra, corners with four equivalent O(3)Er3Ta tetrahedra, corners with four equivalent O(5)Er4 tetrahedra,  an edgeedge with one O(5)Er4 tetrahedra, edges with two equivalent O(3)Er3Ta tetrahedra, and  an edgeedge with one O(1)Er2Ta2 trigonal pyramid. In the fifth O site, O(5) is bonded to two equivalent Er(1) and two equivalent Er(2) atoms to form OEr4 tetrahedra that share corners with two equivalent O(5)Er4 tetrahedra, corners with four equivalent O(3)Er3Ta tetrahedra, corners with four equivalent O(4)Er4 tetrahedra, corners with two equivalent O(1)Er2Ta2 trigonal pyramids,  an edgeedge with one O(4)Er4 tetrahedra, and edges with two equivalent O(3)Er3Ta tetrahedra.
Er3TaO7 crystallizes in the orthorhombic C222_1 space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one O(1), one O(4), one O(5), two equivalent O(2), and two equivalent O(3) atoms to form distorted ErO7 pentagonal bipyramids that share corners with two equivalent Ta(1)O6 octahedra,  a cornercorner with one Er(1)O7 pentagonal bipyramid, edges with two equivalent Ta(1)O6 octahedra, and edges with three equivalent Er(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 39-50°. The Er(1)-O(1) bond length is 2.41 Å. The Er(1)-O(4) bond length is 2.18 Å. The Er(1)-O(5) bond length is 2.27 Å. There is one shorter (2.22 Å) and one longer (2.40 Å) Er(1)-O(2) bond length. There is one shorter (2.23 Å) and one longer (2.38 Å) Er(1)-O(3) bond length. In the second Er site, Er(2) is bonded in a 6-coordinate geometry to two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. Both Er(2)-O(3) bond lengths are 2.31 Å. Both Er(2)-O(4) bond lengths are 2.36 Å. Both Er(2)-O(5) bond lengths are 2.29 Å. Ta(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with four equivalent Er(1)O7 pentagonal bipyramids, and edges with four equivalent Er(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 44°. Both Ta(1)-O(1) bond lengths are 1.98 Å. Both Ta(1)-O(2) bond lengths are 1.92 Å. Both Ta(1)-O(3) bond lengths are 2.03 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Er(1) and two equivalent Ta(1) atoms to form distorted OEr2Ta2 trigonal pyramids that share corners with two equivalent O(5)Er4 tetrahedra, corners with four equivalent O(3)Er3Ta tetrahedra, corners with two equivalent O(1)Er2Ta2 trigonal pyramids,  an edgeedge with one O(4)Er4 tetrahedra, and edges with two equivalent O(3)Er3Ta tetrahedra. In the second O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Er(1) and one Ta(1) atom. In the third O site, O(3) is bonded to one Er(2), two equivalent Er(1), and one Ta(1) atom to form distorted OEr3Ta tetrahedra that share corners with two equivalent O(3)Er3Ta tetrahedra, corners with two equivalent O(4)Er4 tetrahedra, corners with two equivalent O(5)Er4 tetrahedra, corners with two equivalent O(1)Er2Ta2 trigonal pyramids,  an edgeedge with one O(3)Er3Ta tetrahedra,  an edgeedge with one O(4)Er4 tetrahedra,  an edgeedge with one O(5)Er4 tetrahedra, and  an edgeedge with one O(1)Er2Ta2 trigonal pyramid. In the fourth O site, O(4) is bonded to two equivalent Er(1) and two equivalent Er(2) atoms to form OEr4 tetrahedra that share corners with two equivalent O(4)Er4 tetrahedra, corners with four equivalent O(3)Er3Ta tetrahedra, corners with four equivalent O(5)Er4 tetrahedra,  an edgeedge with one O(5)Er4 tetrahedra, edges with two equivalent O(3)Er3Ta tetrahedra, and  an edgeedge with one O(1)Er2Ta2 trigonal pyramid. In the fifth O site, O(5) is bonded to two equivalent Er(1) and two equivalent Er(2) atoms to form OEr4 tetrahedra that share corners with two equivalent O(5)Er4 tetrahedra, corners with four equivalent O(3)Er3Ta tetrahedra, corners with four equivalent O(4)Er4 tetrahedra, corners with two equivalent O(1)Er2Ta2 trigonal pyramids,  an edgeedge with one O(4)Er4 tetrahedra, and edges with two equivalent O(3)Er3Ta tetrahedra.