TbTa3O9 crystallizes in the monoclinic C2/m space group. Tb(1) is bonded to one O(1), two equivalent O(2), two equivalent O(7), three equivalent O(4), and four equivalent O(3) atoms to form TbO12 cuboctahedra that share corners with three equivalent Tb(1)O12 cuboctahedra, faces with two equivalent Tb(1)O12 cuboctahedra, faces with two equivalent Ta(2)O6 octahedra, faces with three equivalent Ta(1)O6 octahedra, and faces with three equivalent Ta(3)O6 octahedra. There are three inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(4), one O(5), two equivalent O(3), and two equivalent O(7) atoms to form TaO6 octahedra that share  a cornercorner with one Ta(1)O6 octahedra, corners with two equivalent Ta(2)O6 octahedra, corners with three equivalent Ta(3)O6 octahedra, and faces with three equivalent Tb(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-18°. In the second Ta site, Ta(2) is bonded to one O(1), one O(6), two equivalent O(2), and two equivalent O(7) atoms to form TaO6 octahedra that share  a cornercorner with one Ta(2)O6 octahedra, corners with two equivalent Ta(1)O6 octahedra, corners with three equivalent Ta(3)O6 octahedra, and faces with two equivalent Tb(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-15°. In the third Ta site, Ta(3) is bonded to one O(4), one O(6), two equivalent O(2), and two equivalent O(3) atoms to form TaO6 octahedra that share corners with three equivalent Ta(1)O6 octahedra, corners with three equivalent Ta(2)O6 octahedra, and faces with three equivalent Tb(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 3-18°. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a linear geometry to two equivalent Tb(1) and two equivalent Ta(2) atoms. In the second O site, O(2) is bonded in a distorted T-shaped geometry to one Tb(1), one Ta(2), and one Ta(3) atom. In the third O site, O(3) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Tb(1), one Ta(1), and one Ta(3) atom. In the fourth O site, O(4) is bonded to three equivalent Tb(1), one Ta(1), and one Ta(3) atom to form a mixture of distorted corner and edge-sharing OTb3Ta2 square pyramids. In the fifth O site, O(5) is bonded in a linear geometry to two equivalent Ta(1) atoms. In the sixth O site, O(6) is bonded in a linear geometry to one Ta(2) and one Ta(3) atom. In the seventh O site, O(7) is bonded in a distorted T-shaped geometry to one Tb(1), one Ta(1), and one Ta(2) atom.
TbTa3O9 crystallizes in the monoclinic C2/m space group. Tb(1) is bonded to one O(1), two equivalent O(2), two equivalent O(7), three equivalent O(4), and four equivalent O(3) atoms to form TbO12 cuboctahedra that share corners with three equivalent Tb(1)O12 cuboctahedra, faces with two equivalent Tb(1)O12 cuboctahedra, faces with two equivalent Ta(2)O6 octahedra, faces with three equivalent Ta(1)O6 octahedra, and faces with three equivalent Ta(3)O6 octahedra. The Tb(1)-O(1) bond length is 2.89 Å. Both Tb(1)-O(2) bond lengths are 2.64 Å. Both Tb(1)-O(7) bond lengths are 2.65 Å. There is one shorter (2.52 Å) and two longer (2.79 Å) Tb(1)-O(4) bond lengths. There are two shorter (2.55 Å) and two longer (2.57 Å) Tb(1)-O(3) bond lengths. There are three inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(4), one O(5), two equivalent O(3), and two equivalent O(7) atoms to form TaO6 octahedra that share  a cornercorner with one Ta(1)O6 octahedra, corners with two equivalent Ta(2)O6 octahedra, corners with three equivalent Ta(3)O6 octahedra, and faces with three equivalent Tb(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-18°. The Ta(1)-O(4) bond length is 2.05 Å. The Ta(1)-O(5) bond length is 1.93 Å. Both Ta(1)-O(3) bond lengths are 2.04 Å. Both Ta(1)-O(7) bond lengths are 1.93 Å. In the second Ta site, Ta(2) is bonded to one O(1), one O(6), two equivalent O(2), and two equivalent O(7) atoms to form TaO6 octahedra that share  a cornercorner with one Ta(2)O6 octahedra, corners with two equivalent Ta(1)O6 octahedra, corners with three equivalent Ta(3)O6 octahedra, and faces with two equivalent Tb(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-15°. The Ta(2)-O(1) bond length is 1.97 Å. The Ta(2)-O(6) bond length is 1.97 Å. Both Ta(2)-O(2) bond lengths are 1.99 Å. Both Ta(2)-O(7) bond lengths are 1.99 Å. In the third Ta site, Ta(3) is bonded to one O(4), one O(6), two equivalent O(2), and two equivalent O(3) atoms to form TaO6 octahedra that share corners with three equivalent Ta(1)O6 octahedra, corners with three equivalent Ta(2)O6 octahedra, and faces with three equivalent Tb(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 3-18°. The Ta(3)-O(4) bond length is 2.08 Å. The Ta(3)-O(6) bond length is 1.91 Å. Both Ta(3)-O(2) bond lengths are 1.94 Å. Both Ta(3)-O(3) bond lengths are 2.04 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a linear geometry to two equivalent Tb(1) and two equivalent Ta(2) atoms. In the second O site, O(2) is bonded in a distorted T-shaped geometry to one Tb(1), one Ta(2), and one Ta(3) atom. In the third O site, O(3) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Tb(1), one Ta(1), and one Ta(3) atom. In the fourth O site, O(4) is bonded to three equivalent Tb(1), one Ta(1), and one Ta(3) atom to form a mixture of distorted corner and edge-sharing OTb3Ta2 square pyramids. In the fifth O site, O(5) is bonded in a linear geometry to two equivalent Ta(1) atoms. In the sixth O site, O(6) is bonded in a linear geometry to one Ta(2) and one Ta(3) atom. In the seventh O site, O(7) is bonded in a distorted T-shaped geometry to one Tb(1), one Ta(1), and one Ta(2) atom.