Eu3Ta5O15 crystallizes in the tetragonal P4/mbm space group. There are two inequivalent Eu sites. In the first Eu site, Eu(1) is bonded in a 7-coordinate geometry to one O(5), two equivalent O(1), and four equivalent O(2) atoms. In the second Eu site, Eu(2) is bonded to four equivalent O(3) and eight equivalent O(4) atoms to form EuO12 cuboctahedra that share faces with two equivalent Eu(2)O12 cuboctahedra and faces with eight equivalent Ta(1)O6 octahedra. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(2), two equivalent O(3), and two equivalent O(4) atoms to form TaO6 octahedra that share  a cornercorner with one Ta(2)O6 octahedra, corners with five equivalent Ta(1)O6 octahedra, and faces with two equivalent Eu(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 3-45°. In the second Ta site, Ta(2) is bonded to two equivalent O(5) and four equivalent O(2) atoms to form corner-sharing TaO6 octahedra. The corner-sharing octahedral tilt angles range from 0-38°. There are five inequivalent O sites. In the first O site, O(4) is bonded in a distorted linear geometry to two equivalent Eu(2) and two equivalent Ta(1) atoms. In the second O site, O(5) is bonded in a linear geometry to two equivalent Eu(1) and two equivalent Ta(2) atoms. In the third O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Eu(1) and two equivalent Ta(1) atoms. In the fourth O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Eu(1), one Ta(1), and one Ta(2) atom. In the fifth O site, O(3) is bonded in a distorted linear geometry to one Eu(2) and two equivalent Ta(1) atoms.
Eu3Ta5O15 crystallizes in the tetragonal P4/mbm space group. There are two inequivalent Eu sites. In the first Eu site, Eu(1) is bonded in a 7-coordinate geometry to one O(5), two equivalent O(1), and four equivalent O(2) atoms. The Eu(1)-O(5) bond length is 2.89 Å. Both Eu(1)-O(1) bond lengths are 2.77 Å. All Eu(1)-O(2) bond lengths are 2.77 Å. In the second Eu site, Eu(2) is bonded to four equivalent O(3) and eight equivalent O(4) atoms to form EuO12 cuboctahedra that share faces with two equivalent Eu(2)O12 cuboctahedra and faces with eight equivalent Ta(1)O6 octahedra. All Eu(2)-O(3) bond lengths are 2.78 Å. All Eu(2)-O(4) bond lengths are 2.73 Å. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(2), two equivalent O(3), and two equivalent O(4) atoms to form TaO6 octahedra that share  a cornercorner with one Ta(2)O6 octahedra, corners with five equivalent Ta(1)O6 octahedra, and faces with two equivalent Eu(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 3-45°. The Ta(1)-O(1) bond length is 2.03 Å. The Ta(1)-O(2) bond length is 2.00 Å. Both Ta(1)-O(3) bond lengths are 1.99 Å. There is one shorter (1.98 Å) and one longer (2.02 Å) Ta(1)-O(4) bond length. In the second Ta site, Ta(2) is bonded to two equivalent O(5) and four equivalent O(2) atoms to form corner-sharing TaO6 octahedra. The corner-sharing octahedral tilt angles range from 0-38°. Both Ta(2)-O(5) bond lengths are 1.99 Å. All Ta(2)-O(2) bond lengths are 2.00 Å. There are five inequivalent O sites. In the first O site, O(4) is bonded in a distorted linear geometry to two equivalent Eu(2) and two equivalent Ta(1) atoms. In the second O site, O(5) is bonded in a linear geometry to two equivalent Eu(1) and two equivalent Ta(2) atoms. In the third O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Eu(1) and two equivalent Ta(1) atoms. In the fourth O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Eu(1), one Ta(1), and one Ta(2) atom. In the fifth O site, O(3) is bonded in a distorted linear geometry to one Eu(2) and two equivalent Ta(1) atoms.