Ta3(Bi2O7)2 crystallizes in the monoclinic C2/c space group. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(2), one O(4), one O(5), one O(7), and two equivalent O(1) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with two equivalent Ta(2)O6 octahedra, corners with two equivalent Bi(1)O7 pentagonal bipyramids, and  an edgeedge with one Bi(1)O7 pentagonal bipyramid. The corner-sharing octahedral tilt angles range from 34-36°. In the second Ta site, Ta(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(7) atoms to form TaO6 octahedra that share corners with four equivalent Ta(1)O6 octahedra, corners with two equivalent Bi(1)O7 pentagonal bipyramids, and edges with two equivalent Bi(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 34-36°. There are two inequivalent Bi sites. In the first Bi site, Bi(1) is bonded to one O(5), one O(6), one O(7), two equivalent O(3), and two equivalent O(4) atoms to form distorted BiO7 pentagonal bipyramids that share  a cornercorner with one Ta(2)O6 octahedra, corners with two equivalent Ta(1)O6 octahedra,  an edgeedge with one Ta(1)O6 octahedra,  an edgeedge with one Ta(2)O6 octahedra, and edges with two equivalent Bi(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 43-44°. In the second Bi site, Bi(2) is bonded in a 8-coordinate geometry to one O(1), one O(2), one O(3), one O(4), two equivalent O(5), and two equivalent O(6) atoms. There are seven inequivalent O sites. In the first O site, O(2) is bonded in a bent 150 degrees geometry to one Ta(1), one Ta(2), and one Bi(2) atom. In the second O site, O(3) is bonded to one Ta(2), one Bi(2), and two equivalent Bi(1) atoms to form a mixture of distorted edge and corner-sharing OTaBi3 tetrahedra. In the third O site, O(4) is bonded to one Ta(1), one Bi(2), and two equivalent Bi(1) atoms to form a mixture of distorted edge and corner-sharing OTaBi3 tetrahedra. In the fourth O site, O(5) is bonded in a 2-coordinate geometry to one Ta(1), one Bi(1), and two equivalent Bi(2) atoms. In the fifth O site, O(6) is bonded in a trigonal non-coplanar geometry to one Bi(1) and two equivalent Bi(2) atoms. In the sixth O site, O(7) is bonded in a 3-coordinate geometry to one Ta(1), one Ta(2), and one Bi(1) atom. In the seventh O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Ta(1) and one Bi(2) atom.
Ta3(Bi2O7)2 crystallizes in the monoclinic C2/c space group. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(2), one O(4), one O(5), one O(7), and two equivalent O(1) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with two equivalent Ta(2)O6 octahedra, corners with two equivalent Bi(1)O7 pentagonal bipyramids, and  an edgeedge with one Bi(1)O7 pentagonal bipyramid. The corner-sharing octahedral tilt angles range from 34-36°. The Ta(1)-O(2) bond length is 2.00 Å. The Ta(1)-O(4) bond length is 2.07 Å. The Ta(1)-O(5) bond length is 1.93 Å. The Ta(1)-O(7) bond length is 2.01 Å. There is one shorter (2.00 Å) and one longer (2.04 Å) Ta(1)-O(1) bond length. In the second Ta site, Ta(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(7) atoms to form TaO6 octahedra that share corners with four equivalent Ta(1)O6 octahedra, corners with two equivalent Bi(1)O7 pentagonal bipyramids, and edges with two equivalent Bi(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 34-36°. Both Ta(2)-O(2) bond lengths are 1.96 Å. Both Ta(2)-O(3) bond lengths are 2.00 Å. Both Ta(2)-O(7) bond lengths are 2.06 Å. There are two inequivalent Bi sites. In the first Bi site, Bi(1) is bonded to one O(5), one O(6), one O(7), two equivalent O(3), and two equivalent O(4) atoms to form distorted BiO7 pentagonal bipyramids that share  a cornercorner with one Ta(2)O6 octahedra, corners with two equivalent Ta(1)O6 octahedra,  an edgeedge with one Ta(1)O6 octahedra,  an edgeedge with one Ta(2)O6 octahedra, and edges with two equivalent Bi(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 43-44°. The Bi(1)-O(5) bond length is 2.28 Å. The Bi(1)-O(6) bond length is 2.29 Å. The Bi(1)-O(7) bond length is 2.48 Å. There is one shorter (2.30 Å) and one longer (2.57 Å) Bi(1)-O(3) bond length. There is one shorter (2.31 Å) and one longer (2.59 Å) Bi(1)-O(4) bond length. In the second Bi site, Bi(2) is bonded in a 8-coordinate geometry to one O(1), one O(2), one O(3), one O(4), two equivalent O(5), and two equivalent O(6) atoms. The Bi(2)-O(1) bond length is 2.59 Å. The Bi(2)-O(2) bond length is 2.76 Å. The Bi(2)-O(3) bond length is 2.58 Å. The Bi(2)-O(4) bond length is 2.41 Å. There is one shorter (2.74 Å) and one longer (2.85 Å) Bi(2)-O(5) bond length. Both Bi(2)-O(6) bond lengths are 2.22 Å. There are seven inequivalent O sites. In the first O site, O(2) is bonded in a bent 150 degrees geometry to one Ta(1), one Ta(2), and one Bi(2) atom. In the second O site, O(3) is bonded to one Ta(2), one Bi(2), and two equivalent Bi(1) atoms to form a mixture of distorted edge and corner-sharing OTaBi3 tetrahedra. In the third O site, O(4) is bonded to one Ta(1), one Bi(2), and two equivalent Bi(1) atoms to form a mixture of distorted edge and corner-sharing OTaBi3 tetrahedra. In the fourth O site, O(5) is bonded in a 2-coordinate geometry to one Ta(1), one Bi(1), and two equivalent Bi(2) atoms. In the fifth O site, O(6) is bonded in a trigonal non-coplanar geometry to one Bi(1) and two equivalent Bi(2) atoms. In the sixth O site, O(7) is bonded in a 3-coordinate geometry to one Ta(1), one Ta(2), and one Bi(1) atom. In the seventh O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Ta(1) and one Bi(2) atom.