Tb3NbO7 crystallizes in the orthorhombic C222_1 space group. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to one O(1), one O(2), one O(4), two equivalent O(3), and two equivalent O(5) atoms to form distorted TbO7 pentagonal bipyramids that share corners with two equivalent Nb(1)O6 octahedra,  a cornercorner with one Tb(1)O7 pentagonal bipyramid, edges with two equivalent Nb(1)O6 octahedra, and edges with three equivalent Tb(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 38-50°. In the second Tb site, Tb(2) is bonded in a 6-coordinate geometry to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms. Nb(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(5) atoms to form NbO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra, corners with four equivalent Tb(1)O7 pentagonal bipyramids, and edges with four equivalent Tb(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 Tb(1) and two equivalent Nb(1) atoms to form distorted OTb2Nb2 trigonal pyramids that share corners with two equivalent O(4)Tb4 tetrahedra, corners with four equivalent O(5)Tb3Nb tetrahedra, corners with two equivalent O(1)Tb2Nb2 trigonal pyramids,  an edgeedge with one O(2)Tb4 tetrahedra, and edges with two equivalent O(5)Tb3Nb tetrahedra. In the second O site, O(2) is bonded to two equivalent Tb(1) and two equivalent Tb(2) atoms to form OTb4 tetrahedra that share corners with two equivalent O(2)Tb4 tetrahedra, corners with four equivalent O(5)Tb3Nb tetrahedra, corners with four equivalent O(4)Tb4 tetrahedra,  an edgeedge with one O(4)Tb4 tetrahedra, edges with two equivalent O(5)Tb3Nb tetrahedra, and  an edgeedge with one O(1)Tb2Nb2 trigonal pyramid. In the third O site, O(3) is bonded in a 3-coordinate geometry to two equivalent Tb(1) and one Nb(1) atom. In the fourth O site, O(4) is bonded to two equivalent Tb(1) and two equivalent Tb(2) atoms to form OTb4 tetrahedra that share corners with two equivalent O(4)Tb4 tetrahedra, corners with four equivalent O(5)Tb3Nb tetrahedra, corners with four equivalent O(2)Tb4 tetrahedra, corners with two equivalent O(1)Tb2Nb2 trigonal pyramids,  an edgeedge with one O(2)Tb4 tetrahedra, and edges with two equivalent O(5)Tb3Nb tetrahedra. In the fifth O site, O(5) is bonded to one Tb(2), two equivalent Tb(1), and one Nb(1) atom to form OTb3Nb tetrahedra that share corners with two equivalent O(5)Tb3Nb tetrahedra, corners with two equivalent O(2)Tb4 tetrahedra, corners with two equivalent O(4)Tb4 tetrahedra, corners with two equivalent O(1)Tb2Nb2 trigonal pyramids,  an edgeedge with one O(5)Tb3Nb tetrahedra,  an edgeedge with one O(2)Tb4 tetrahedra,  an edgeedge with one O(4)Tb4 tetrahedra, and  an edgeedge with one O(1)Tb2Nb2 trigonal pyramid.
Tb3NbO7 crystallizes in the orthorhombic C222_1 space group. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to one O(1), one O(2), one O(4), two equivalent O(3), and two equivalent O(5) atoms to form distorted TbO7 pentagonal bipyramids that share corners with two equivalent Nb(1)O6 octahedra,  a cornercorner with one Tb(1)O7 pentagonal bipyramid, edges with two equivalent Nb(1)O6 octahedra, and edges with three equivalent Tb(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 38-50°. The Tb(1)-O(1) bond length is 2.44 Å. The Tb(1)-O(2) bond length is 2.22 Å. The Tb(1)-O(4) bond length is 2.30 Å. There is one shorter (2.27 Å) and one longer (2.44 Å) Tb(1)-O(3) bond length. There is one shorter (2.28 Å) and one longer (2.41 Å) Tb(1)-O(5) bond length. In the second Tb site, Tb(2) is bonded in a 6-coordinate geometry to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms. Both Tb(2)-O(2) bond lengths are 2.39 Å. Both Tb(2)-O(4) bond lengths are 2.33 Å. Both Tb(2)-O(5) bond lengths are 2.34 Å. Nb(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(5) atoms to form NbO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra, corners with four equivalent Tb(1)O7 pentagonal bipyramids, and edges with four equivalent Tb(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 44°. Both Nb(1)-O(1) bond lengths are 2.02 Å. Both Nb(1)-O(3) bond lengths are 1.93 Å. Both Nb(1)-O(5) bond lengths are 2.06 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Tb(1) and two equivalent Nb(1) atoms to form distorted OTb2Nb2 trigonal pyramids that share corners with two equivalent O(4)Tb4 tetrahedra, corners with four equivalent O(5)Tb3Nb tetrahedra, corners with two equivalent O(1)Tb2Nb2 trigonal pyramids,  an edgeedge with one O(2)Tb4 tetrahedra, and edges with two equivalent O(5)Tb3Nb tetrahedra. In the second O site, O(2) is bonded to two equivalent Tb(1) and two equivalent Tb(2) atoms to form OTb4 tetrahedra that share corners with two equivalent O(2)Tb4 tetrahedra, corners with four equivalent O(5)Tb3Nb tetrahedra, corners with four equivalent O(4)Tb4 tetrahedra,  an edgeedge with one O(4)Tb4 tetrahedra, edges with two equivalent O(5)Tb3Nb tetrahedra, and  an edgeedge with one O(1)Tb2Nb2 trigonal pyramid. In the third O site, O(3) is bonded in a 3-coordinate geometry to two equivalent Tb(1) and one Nb(1) atom. In the fourth O site, O(4) is bonded to two equivalent Tb(1) and two equivalent Tb(2) atoms to form OTb4 tetrahedra that share corners with two equivalent O(4)Tb4 tetrahedra, corners with four equivalent O(5)Tb3Nb tetrahedra, corners with four equivalent O(2)Tb4 tetrahedra, corners with two equivalent O(1)Tb2Nb2 trigonal pyramids,  an edgeedge with one O(2)Tb4 tetrahedra, and edges with two equivalent O(5)Tb3Nb tetrahedra. In the fifth O site, O(5) is bonded to one Tb(2), two equivalent Tb(1), and one Nb(1) atom to form OTb3Nb tetrahedra that share corners with two equivalent O(5)Tb3Nb tetrahedra, corners with two equivalent O(2)Tb4 tetrahedra, corners with two equivalent O(4)Tb4 tetrahedra, corners with two equivalent O(1)Tb2Nb2 trigonal pyramids,  an edgeedge with one O(5)Tb3Nb tetrahedra,  an edgeedge with one O(2)Tb4 tetrahedra,  an edgeedge with one O(4)Tb4 tetrahedra, and  an edgeedge with one O(1)Tb2Nb2 trigonal pyramid.