Ba3Ta4NbO15 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 Ta(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 one O(2), one O(4), two equivalent O(3), and two equivalent O(5) atoms to form TaO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra, corners with five equivalent Ta(1)O6 octahedra, and faces with two equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 3-39°. Nb(1) is bonded to two equivalent O(1) and four equivalent O(4) atoms to form NbO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra and corners with four equivalent Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 0-36°. 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 Ta(1) atoms. In the second O site, O(1) is bonded in a linear geometry to two equivalent Nb(1) atoms. In the third O site, O(2) is bonded in a 4-coordinate geometry to two equivalent Ba(2) and two equivalent Ta(1) atoms. In the fourth O site, O(3) is bonded in a distorted linear geometry to one Ba(1) and two equivalent Ta(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.
Ba3Ta4NbO15 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 Ta(1)O6 octahedra. All Ba(1)-O(3) bond lengths are 2.85 Å. All Ba(1)-O(5) bond lengths are 2.83 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 2.89 Å. Ta(1) is bonded to one O(2), one O(4), two equivalent O(3), and two equivalent O(5) atoms to form TaO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra, corners with five equivalent Ta(1)O6 octahedra, and faces with two equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 3-39°. The Ta(1)-O(2) bond length is 2.02 Å. The Ta(1)-O(4) bond length is 1.95 Å. Both Ta(1)-O(3) bond lengths are 2.01 Å. There is one shorter (2.03 Å) and one longer (2.06 Å) Ta(1)-O(5) bond length. Nb(1) is bonded to two equivalent O(1) and four equivalent O(4) atoms to form NbO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra and corners with four equivalent Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 0-36°. Both Nb(1)-O(1) bond lengths are 2.01 Å. All Nb(1)-O(4) bond lengths are 2.03 Å. 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 Ta(1) atoms. In the second O site, O(1) is bonded in a linear geometry to two equivalent Nb(1) atoms. In the third O site, O(2) is bonded in a 4-coordinate geometry to two equivalent Ba(2) and two equivalent Ta(1) atoms. In the fourth O site, O(3) is bonded in a distorted linear geometry to one Ba(1) and two equivalent Ta(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.