Ba3TaNb4O15 crystallizes in the tetragonal P4/mbm space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to four equivalent O(3) and eight equivalent O(5) atoms to form BaO12 cuboctahedra that share faces with two equivalent Ba(1)O12 cuboctahedra and faces with eight equivalent Nb(1)O6 octahedra. In the second Ba site, Ba(2) is bonded in a 6-coordinate geometry to two equivalent O(2) and four equivalent O(4) atoms. Ta(1) is bonded to two equivalent O(1) and four equivalent O(4) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra and corners with four equivalent Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 0-35°. Nb(1) is bonded to one O(2), one O(4), two equivalent O(3), and two equivalent O(5) atoms to form NbO6 octahedra that share  a cornercorner with one Ta(1)O6 octahedra, corners with five equivalent Nb(1)O6 octahedra, and faces with two equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 3-39°. There are five inequivalent O sites. In the first O site, O(5) is bonded in a distorted linear geometry to two equivalent Ba(1) and two equivalent Nb(1) atoms. In the second O site, O(1) is bonded in a linear geometry to two equivalent Ta(1) atoms. In the third O site, O(2) is bonded in a 4-coordinate geometry to two equivalent Ba(2) and two equivalent Nb(1) atoms. In the fourth O site, O(3) is bonded in a distorted linear geometry to one Ba(1) and two equivalent Nb(1) atoms. In the fifth O site, O(4) is bonded in a distorted bent 150 degrees geometry to two equivalent Ba(2), one Ta(1), and one Nb(1) atom.
Ba3TaNb4O15 crystallizes in the tetragonal P4/mbm space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to four equivalent O(3) and eight equivalent O(5) atoms to form BaO12 cuboctahedra that share faces with two equivalent Ba(1)O12 cuboctahedra and faces with eight equivalent Nb(1)O6 octahedra. All Ba(1)-O(3) bond lengths are 2.86 Å. All Ba(1)-O(5) bond lengths are 2.84 Å. In the second Ba site, Ba(2) is bonded in a 6-coordinate geometry to two equivalent O(2) and four equivalent O(4) atoms. Both Ba(2)-O(2) bond lengths are 2.86 Å. All Ba(2)-O(4) bond lengths are 2.91 Å. Ta(1) is bonded to two equivalent O(1) and four equivalent O(4) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra and corners with four equivalent Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 0-35°. Both Ta(1)-O(1) bond lengths are 2.02 Å. All Ta(1)-O(4) bond lengths are 1.99 Å. Nb(1) is bonded to one O(2), one O(4), two equivalent O(3), and two equivalent O(5) atoms to form NbO6 octahedra that share  a cornercorner with one Ta(1)O6 octahedra, corners with five equivalent Nb(1)O6 octahedra, and faces with two equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 3-39°. The Nb(1)-O(2) bond length is 2.02 Å. The Nb(1)-O(4) bond length is 2.00 Å. Both Nb(1)-O(3) bond lengths are 2.02 Å. There is one shorter (2.01 Å) and one longer (2.10 Å) Nb(1)-O(5) bond length. There are five inequivalent O sites. In the first O site, O(5) is bonded in a distorted linear geometry to two equivalent Ba(1) and two equivalent Nb(1) atoms. In the second O site, O(1) is bonded in a linear geometry to two equivalent Ta(1) atoms. In the third O site, O(2) is bonded in a 4-coordinate geometry to two equivalent Ba(2) and two equivalent Nb(1) atoms. In the fourth O site, O(3) is bonded in a distorted linear geometry to one Ba(1) and two equivalent Nb(1) atoms. In the fifth O site, O(4) is bonded in a distorted bent 150 degrees geometry to two equivalent Ba(2), one Ta(1), and one Nb(1) atom.