LaTa3O9 crystallizes in the monoclinic C2/m space group. La(1) is bonded to one O(3), two equivalent O(1), two equivalent O(5), three equivalent O(7), and four equivalent O(4) atoms to form LaO12 cuboctahedra that share corners with three equivalent La(1)O12 cuboctahedra, faces with two equivalent La(1)O12 cuboctahedra, faces with two equivalent Ta(1)O6 octahedra, faces with three equivalent Ta(2)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(2), one O(3), two equivalent O(1), and two equivalent O(5) atoms to form TaO6 octahedra that share  a cornercorner with one Ta(1)O6 octahedra, corners with two equivalent Ta(3)O6 octahedra, corners with three equivalent Ta(2)O6 octahedra, and faces with two equivalent La(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-13°. In the second Ta site, Ta(2) is bonded to one O(2), one O(7), two equivalent O(1), and two equivalent O(4) atoms to form TaO6 octahedra that share corners with three equivalent Ta(1)O6 octahedra, corners with three equivalent Ta(3)O6 octahedra, and faces with three equivalent La(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 2-15°. In the third Ta site, Ta(3) is bonded to one O(6), one O(7), two equivalent O(4), and two equivalent O(5) atoms to form TaO6 octahedra that share  a cornercorner with one Ta(3)O6 octahedra, corners with two equivalent Ta(1)O6 octahedra, corners with three equivalent Ta(2)O6 octahedra, and faces with three equivalent La(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-15°. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one La(1), one Ta(1), and one Ta(2) atom. In the second O site, O(2) is bonded in a linear geometry to one Ta(1) and one Ta(2) atom. In the third O site, O(3) is bonded in a distorted linear geometry to two equivalent La(1) and two equivalent Ta(1) atoms. In the fourth O site, O(4) is bonded in a distorted rectangular see-saw-like geometry to two equivalent La(1), one Ta(2), and one Ta(3) atom. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one La(1), one Ta(1), and one Ta(3) atom. In the sixth O site, O(6) is bonded in a linear geometry to two equivalent Ta(3) atoms. In the seventh O site, O(7) is bonded to three equivalent La(1), one Ta(2), and one Ta(3) atom to form a mixture of distorted corner and edge-sharing OLa3Ta2 square pyramids.
LaTa3O9 crystallizes in the monoclinic C2/m space group. La(1) is bonded to one O(3), two equivalent O(1), two equivalent O(5), three equivalent O(7), and four equivalent O(4) atoms to form LaO12 cuboctahedra that share corners with three equivalent La(1)O12 cuboctahedra, faces with two equivalent La(1)O12 cuboctahedra, faces with two equivalent Ta(1)O6 octahedra, faces with three equivalent Ta(2)O6 octahedra, and faces with three equivalent Ta(3)O6 octahedra. The La(1)-O(3) bond length is 2.80 Å. Both La(1)-O(1) bond lengths are 2.65 Å. Both La(1)-O(5) bond lengths are 2.65 Å. There is one shorter (2.62 Å) and two longer (2.78 Å) La(1)-O(7) bond lengths. There are two shorter (2.66 Å) and two longer (2.67 Å) La(1)-O(4) bond lengths. There are three inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(2), one O(3), two equivalent O(1), and two equivalent O(5) atoms to form TaO6 octahedra that share  a cornercorner with one Ta(1)O6 octahedra, corners with two equivalent Ta(3)O6 octahedra, corners with three equivalent Ta(2)O6 octahedra, and faces with two equivalent La(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-13°. The Ta(1)-O(2) bond length is 1.94 Å. The Ta(1)-O(3) bond length is 2.00 Å. Both Ta(1)-O(1) bond lengths are 1.98 Å. Both Ta(1)-O(5) bond lengths are 1.98 Å. In the second Ta site, Ta(2) is bonded to one O(2), one O(7), two equivalent O(1), and two equivalent O(4) atoms to form TaO6 octahedra that share corners with three equivalent Ta(1)O6 octahedra, corners with three equivalent Ta(3)O6 octahedra, and faces with three equivalent La(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 2-15°. The Ta(2)-O(2) bond length is 1.89 Å. The Ta(2)-O(7) bond length is 2.12 Å. Both Ta(2)-O(1) bond lengths are 1.94 Å. Both Ta(2)-O(4) bond lengths are 2.02 Å. In the third Ta site, Ta(3) is bonded to one O(6), one O(7), two equivalent O(4), and two equivalent O(5) atoms to form TaO6 octahedra that share  a cornercorner with one Ta(3)O6 octahedra, corners with two equivalent Ta(1)O6 octahedra, corners with three equivalent Ta(2)O6 octahedra, and faces with three equivalent La(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-15°. The Ta(3)-O(6) bond length is 1.91 Å. The Ta(3)-O(7) bond length is 2.07 Å. Both Ta(3)-O(4) bond lengths are 2.03 Å. Both Ta(3)-O(5) bond lengths are 1.93 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one La(1), one Ta(1), and one Ta(2) atom. In the second O site, O(2) is bonded in a linear geometry to one Ta(1) and one Ta(2) atom. In the third O site, O(3) is bonded in a distorted linear geometry to two equivalent La(1) and two equivalent Ta(1) atoms. In the fourth O site, O(4) is bonded in a distorted rectangular see-saw-like geometry to two equivalent La(1), one Ta(2), and one Ta(3) atom. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one La(1), one Ta(1), and one Ta(3) atom. In the sixth O site, O(6) is bonded in a linear geometry to two equivalent Ta(3) atoms. In the seventh O site, O(7) is bonded to three equivalent La(1), one Ta(2), and one Ta(3) atom to form a mixture of distorted corner and edge-sharing OLa3Ta2 square pyramids.