EuTa3O9 crystallizes in the monoclinic C2/m space group. Eu(1) is bonded to one O(7), two equivalent O(2), two equivalent O(3), three equivalent O(6), and four equivalent O(1) atoms to form EuO12 cuboctahedra that share corners with three equivalent Eu(1)O12 cuboctahedra, faces with two equivalent Eu(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(6), two equivalent O(1), and two equivalent O(2) 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 Eu(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-16°. In the second Ta site, Ta(2) is bonded to one O(5), one O(7), two equivalent O(2), and two equivalent O(3) 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 Eu(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-14°. In the third Ta site, Ta(3) is bonded to one O(5), one O(6), two equivalent O(1), and two equivalent O(3) atoms to form distorted 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 Eu(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 2-16°. There are seven inequivalent O sites. In the first O site, O(7) is bonded in a linear geometry to two equivalent Eu(1) and two equivalent Ta(2) atoms. In the second O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Eu(1), one Ta(1), and one Ta(3) atom. In the third O site, O(2) is bonded in a distorted T-shaped geometry to one Eu(1), one Ta(1), and one Ta(2) atom. In the fourth O site, O(3) is bonded in a distorted T-shaped geometry to one Eu(1), one Ta(2), and one Ta(3) atom. In the fifth O site, O(4) is bonded in a linear geometry to two equivalent Ta(1) atoms. In the sixth O site, O(5) is bonded in a linear geometry to one Ta(2) and one Ta(3) atom. In the seventh O site, O(6) is bonded to three equivalent Eu(1), one Ta(1), and one Ta(3) atom to form a mixture of distorted corner and edge-sharing OEu3Ta2 square pyramids.
EuTa3O9 crystallizes in the monoclinic C2/m space group. Eu(1) is bonded to one O(7), two equivalent O(2), two equivalent O(3), three equivalent O(6), and four equivalent O(1) atoms to form EuO12 cuboctahedra that share corners with three equivalent Eu(1)O12 cuboctahedra, faces with two equivalent Eu(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 Eu(1)-O(7) bond length is 2.82 Å. Both Eu(1)-O(2) bond lengths are 2.64 Å. Both Eu(1)-O(3) bond lengths are 2.64 Å. There is one shorter (2.52 Å) and two longer (2.77 Å) Eu(1)-O(6) bond lengths. There are two shorter (2.63 Å) and two longer (2.64 Å) Eu(1)-O(1) bond lengths. There are three inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(4), one O(6), two equivalent O(1), and two equivalent O(2) 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 Eu(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-16°. The Ta(1)-O(4) bond length is 1.91 Å. The Ta(1)-O(6) bond length is 2.07 Å. Both Ta(1)-O(1) bond lengths are 2.02 Å. Both Ta(1)-O(2) bond lengths are 1.92 Å. In the second Ta site, Ta(2) is bonded to one O(5), one O(7), two equivalent O(2), and two equivalent O(3) 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 Eu(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-14°. The Ta(2)-O(5) bond length is 1.95 Å. The Ta(2)-O(7) bond length is 1.98 Å. Both Ta(2)-O(2) bond lengths are 1.98 Å. Both Ta(2)-O(3) bond lengths are 1.97 Å. In the third Ta site, Ta(3) is bonded to one O(5), one O(6), two equivalent O(1), and two equivalent O(3) atoms to form distorted 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 Eu(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 2-16°. The Ta(3)-O(5) bond length is 1.89 Å. The Ta(3)-O(6) bond length is 2.12 Å. Both Ta(3)-O(1) bond lengths are 2.01 Å. Both Ta(3)-O(3) bond lengths are 1.93 Å. There are seven inequivalent O sites. In the first O site, O(7) is bonded in a linear geometry to two equivalent Eu(1) and two equivalent Ta(2) atoms. In the second O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Eu(1), one Ta(1), and one Ta(3) atom. In the third O site, O(2) is bonded in a distorted T-shaped geometry to one Eu(1), one Ta(1), and one Ta(2) atom. In the fourth O site, O(3) is bonded in a distorted T-shaped geometry to one Eu(1), one Ta(2), and one Ta(3) atom. In the fifth O site, O(4) is bonded in a linear geometry to two equivalent Ta(1) atoms. In the sixth O site, O(5) is bonded in a linear geometry to one Ta(2) and one Ta(3) atom. In the seventh O site, O(6) is bonded to three equivalent Eu(1), one Ta(1), and one Ta(3) atom to form a mixture of distorted corner and edge-sharing OEu3Ta2 square pyramids.