Tb3NbO7 crystallizes in the orthorhombic Pnma space group. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded in a 6-coordinate geometry to two equivalent O(1) and four equivalent O(4) atoms. In the second Tb site, Tb(2) is bonded to one O(3), two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted TbO7 pentagonal bipyramids that share corners with two equivalent Nb(1)O6 octahedra, corners with three equivalent Tb(2)O7 pentagonal bipyramids, edges with two equivalent Nb(1)O6 octahedra, and edges with two equivalent Tb(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 33-45°. Nb(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form NbO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra, corners with four equivalent Tb(2)O7 pentagonal bipyramids, and edges with four equivalent Tb(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 40°. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Tb(1), two equivalent Tb(2), and one Nb(1) atom to form OTb3Nb tetrahedra that share corners with four equivalent O(4)Tb4 tetrahedra, edges with two equivalent O(1)Tb3Nb tetrahedra, and edges with two equivalent O(4)Tb4 tetrahedra. In the second O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Tb(2) and one Nb(1) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Tb(2) and two equivalent Nb(1) atoms. 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 four equivalent O(1)Tb3Nb tetrahedra, corners with six equivalent O(4)Tb4 tetrahedra,  an edgeedge with one O(4)Tb4 tetrahedra, and edges with two equivalent O(1)Tb3Nb tetrahedra.
Tb3NbO7 crystallizes in the orthorhombic Pnma space group. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded in a 6-coordinate geometry to two equivalent O(1) and four equivalent O(4) atoms. Both Tb(1)-O(1) bond lengths are 2.31 Å. There are two shorter (2.36 Å) and two longer (2.42 Å) Tb(1)-O(4) bond lengths. In the second Tb site, Tb(2) is bonded to one O(3), two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted TbO7 pentagonal bipyramids that share corners with two equivalent Nb(1)O6 octahedra, corners with three equivalent Tb(2)O7 pentagonal bipyramids, edges with two equivalent Nb(1)O6 octahedra, and edges with two equivalent Tb(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 33-45°. The Tb(2)-O(3) bond length is 2.62 Å. There is one shorter (2.31 Å) and one longer (2.42 Å) Tb(2)-O(1) bond length. There is one shorter (2.30 Å) and one longer (2.42 Å) Tb(2)-O(2) bond length. There is one shorter (2.27 Å) and one longer (2.31 Å) Tb(2)-O(4) bond length. Nb(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form NbO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra, corners with four equivalent Tb(2)O7 pentagonal bipyramids, and edges with four equivalent Tb(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 40°. Both Nb(1)-O(1) bond lengths are 2.09 Å. Both Nb(1)-O(2) bond lengths are 1.94 Å. There is one shorter (1.95 Å) and one longer (2.04 Å) Nb(1)-O(3) bond length. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Tb(1), two equivalent Tb(2), and one Nb(1) atom to form OTb3Nb tetrahedra that share corners with four equivalent O(4)Tb4 tetrahedra, edges with two equivalent O(1)Tb3Nb tetrahedra, and edges with two equivalent O(4)Tb4 tetrahedra. In the second O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Tb(2) and one Nb(1) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Tb(2) and two equivalent Nb(1) atoms. 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 four equivalent O(1)Tb3Nb tetrahedra, corners with six equivalent O(4)Tb4 tetrahedra,  an edgeedge with one O(4)Tb4 tetrahedra, and edges with two equivalent O(1)Tb3Nb tetrahedra.