Eu2Ta2O7 crystallizes in the orthorhombic Cmc2_1 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(3), three equivalent O(2), and three equivalent O(5) atoms. In the second Eu site, Eu(2) is bonded to one O(3), two equivalent O(1), two equivalent O(4), three equivalent O(7), and four equivalent O(6) atoms to form distorted EuO12 cuboctahedra that share corners with six equivalent Eu(2)O12 cuboctahedra, faces with four equivalent Eu(2)O12 cuboctahedra, faces with two equivalent Ta(1)O6 octahedra, and faces with six equivalent Ta(2)O6 octahedra. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(2), one O(4), one O(5), and two equivalent O(3) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with two equivalent Ta(2)O6 octahedra, and faces with two equivalent Eu(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 12-32°. In the second Ta site, Ta(2) is bonded to one O(1), one O(4), two equivalent O(6), and two equivalent O(7) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with four equivalent Ta(2)O6 octahedra, and faces with six equivalent Eu(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 9-32°. There are seven inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Eu(2), one Ta(1), and one Ta(2) atom to form distorted OEu2Ta2 tetrahedra that share  a cornercorner with one O(5)Eu3Ta tetrahedra and corners with two equivalent O(1)Eu2Ta2 tetrahedra. In the second O site, O(2) is bonded in a 4-coordinate geometry to three equivalent Eu(1) and one Ta(1) atom. In the third O site, O(3) is bonded in a 2-coordinate geometry to one Eu(1), one Eu(2), and two equivalent Ta(1) atoms. In the fourth O site, O(4) is bonded in a distorted linear geometry to two equivalent Eu(2), one Ta(1), and one Ta(2) atom. In the fifth O site, O(5) is bonded to three equivalent Eu(1) and one Ta(1) atom to form OEu3Ta tetrahedra that share  a cornercorner with one O(1)Eu2Ta2 tetrahedra and corners with six equivalent O(5)Eu3Ta tetrahedra. In the sixth O site, O(6) is bonded in a 6-coordinate geometry to four equivalent Eu(2) and two equivalent Ta(2) atoms. In the seventh O site, O(7) is bonded in a 2-coordinate geometry to three equivalent Eu(2) and two equivalent Ta(2) atoms.
Eu2Ta2O7 crystallizes in the orthorhombic Cmc2_1 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(3), three equivalent O(2), and three equivalent O(5) atoms. The Eu(1)-O(3) bond length is 2.97 Å. There are two shorter (2.52 Å) and one longer (2.77 Å) Eu(1)-O(2) bond length. There is one shorter (2.44 Å) and two longer (2.46 Å) Eu(1)-O(5) bond lengths. In the second Eu site, Eu(2) is bonded to one O(3), two equivalent O(1), two equivalent O(4), three equivalent O(7), and four equivalent O(6) atoms to form distorted EuO12 cuboctahedra that share corners with six equivalent Eu(2)O12 cuboctahedra, faces with four equivalent Eu(2)O12 cuboctahedra, faces with two equivalent Ta(1)O6 octahedra, and faces with six equivalent Ta(2)O6 octahedra. The Eu(2)-O(3) bond length is 2.66 Å. Both Eu(2)-O(1) bond lengths are 2.53 Å. Both Eu(2)-O(4) bond lengths are 3.01 Å. There are a spread of Eu(2)-O(7) bond distances ranging from 2.73-2.96 Å. There are two shorter (2.72 Å) and two longer (2.95 Å) Eu(2)-O(6) bond lengths. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(2), one O(4), one O(5), and two equivalent O(3) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with two equivalent Ta(2)O6 octahedra, and faces with two equivalent Eu(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 12-32°. The Ta(1)-O(1) bond length is 2.07 Å. The Ta(1)-O(2) bond length is 1.87 Å. The Ta(1)-O(4) bond length is 2.24 Å. The Ta(1)-O(5) bond length is 1.93 Å. Both Ta(1)-O(3) bond lengths are 2.02 Å. In the second Ta site, Ta(2) is bonded to one O(1), one O(4), two equivalent O(6), and two equivalent O(7) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with four equivalent Ta(2)O6 octahedra, and faces with six equivalent Eu(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 9-32°. The Ta(2)-O(1) bond length is 2.00 Å. The Ta(2)-O(4) bond length is 1.88 Å. There is one shorter (2.01 Å) and one longer (2.18 Å) Ta(2)-O(6) bond length. Both Ta(2)-O(7) bond lengths are 2.01 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Eu(2), one Ta(1), and one Ta(2) atom to form distorted OEu2Ta2 tetrahedra that share  a cornercorner with one O(5)Eu3Ta tetrahedra and corners with two equivalent O(1)Eu2Ta2 tetrahedra. In the second O site, O(2) is bonded in a 4-coordinate geometry to three equivalent Eu(1) and one Ta(1) atom. In the third O site, O(3) is bonded in a 2-coordinate geometry to one Eu(1), one Eu(2), and two equivalent Ta(1) atoms. In the fourth O site, O(4) is bonded in a distorted linear geometry to two equivalent Eu(2), one Ta(1), and one Ta(2) atom. In the fifth O site, O(5) is bonded to three equivalent Eu(1) and one Ta(1) atom to form OEu3Ta tetrahedra that share  a cornercorner with one O(1)Eu2Ta2 tetrahedra and corners with six equivalent O(5)Eu3Ta tetrahedra. In the sixth O site, O(6) is bonded in a 6-coordinate geometry to four equivalent Eu(2) and two equivalent Ta(2) atoms. In the seventh O site, O(7) is bonded in a 2-coordinate geometry to three equivalent Eu(2) and two equivalent Ta(2) atoms.