Sr6Ta2O11 crystallizes in the tetragonal P4/nmm space group. There are three inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 9-coordinate geometry to one O(4), two equivalent O(2), two equivalent O(3), and four equivalent O(1) atoms. In the second Sr site, Sr(2) is bonded to one O(4) and four equivalent O(1) atoms to form distorted SrO5 square pyramids that share corners with four equivalent Ta(2)O6 octahedra and  a cornercorner with one Ta(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 10°. In the third Sr site, Sr(3) is bonded in a 6-coordinate geometry to two equivalent O(2) and four equivalent O(3) atoms. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(4) and four equivalent O(3) atoms to form distorted TaO5 trigonal bipyramids that share  a cornercorner with one Sr(2)O5 square pyramid. In the second Ta site, Ta(2) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form TaO6 octahedra that share corners with four equivalent Sr(2)O5 square pyramids. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Sr(2), four equivalent Sr(1), and one Ta(2) atom to form distorted OSr5Ta octahedra that share corners with two equivalent O(1)Sr5Ta octahedra, corners with six equivalent O(3)Sr3Ta tetrahedra, edges with two equivalent O(1)Sr5Ta octahedra,  an edgeedge with one O(3)Sr3Ta tetrahedra, and faces with four equivalent O(1)Sr5Ta octahedra. The corner-sharing octahedral tilt angles range from 4-16°. In the second O site, O(2) is bonded in a distorted linear geometry to one Sr(3), four equivalent Sr(1), and one Ta(2) atom. In the third O site, O(3) is bonded to one Sr(3), two equivalent Sr(1), and one Ta(1) atom to form distorted OSr3Ta tetrahedra that share corners with six equivalent O(1)Sr5Ta octahedra, corners with four equivalent O(3)Sr3Ta tetrahedra,  an edgeedge with one O(1)Sr5Ta octahedra, and edges with two equivalent O(3)Sr3Ta tetrahedra. The corner-sharing octahedral tilt angles range from 11-70°. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to one Sr(2), four equivalent Sr(1), and one Ta(1) atom.
Sr6Ta2O11 crystallizes in the tetragonal P4/nmm space group. There are three inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 9-coordinate geometry to one O(4), two equivalent O(2), two equivalent O(3), and four equivalent O(1) atoms. The Sr(1)-O(4) bond length is 3.10 Å. Both Sr(1)-O(2) bond lengths are 3.03 Å. Both Sr(1)-O(3) bond lengths are 2.64 Å. There are two shorter (2.82 Å) and two longer (2.92 Å) Sr(1)-O(1) bond lengths. In the second Sr site, Sr(2) is bonded to one O(4) and four equivalent O(1) atoms to form distorted SrO5 square pyramids that share corners with four equivalent Ta(2)O6 octahedra and  a cornercorner with one Ta(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 10°. The Sr(2)-O(4) bond length is 2.85 Å. All Sr(2)-O(1) bond lengths are 2.30 Å. In the third Sr site, Sr(3) is bonded in a 6-coordinate geometry to two equivalent O(2) and four equivalent O(3) atoms. Both Sr(3)-O(2) bond lengths are 2.32 Å. All Sr(3)-O(3) bond lengths are 2.57 Å. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(4) and four equivalent O(3) atoms to form distorted TaO5 trigonal bipyramids that share  a cornercorner with one Sr(2)O5 square pyramid. The Ta(1)-O(4) bond length is 1.91 Å. All Ta(1)-O(3) bond lengths are 1.97 Å. In the second Ta site, Ta(2) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form TaO6 octahedra that share corners with four equivalent Sr(2)O5 square pyramids. Both Ta(2)-O(2) bond lengths are 1.95 Å. All Ta(2)-O(1) bond lengths are 2.01 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Sr(2), four equivalent Sr(1), and one Ta(2) atom to form distorted OSr5Ta octahedra that share corners with two equivalent O(1)Sr5Ta octahedra, corners with six equivalent O(3)Sr3Ta tetrahedra, edges with two equivalent O(1)Sr5Ta octahedra,  an edgeedge with one O(3)Sr3Ta tetrahedra, and faces with four equivalent O(1)Sr5Ta octahedra. The corner-sharing octahedral tilt angles range from 4-16°. In the second O site, O(2) is bonded in a distorted linear geometry to one Sr(3), four equivalent Sr(1), and one Ta(2) atom. In the third O site, O(3) is bonded to one Sr(3), two equivalent Sr(1), and one Ta(1) atom to form distorted OSr3Ta tetrahedra that share corners with six equivalent O(1)Sr5Ta octahedra, corners with four equivalent O(3)Sr3Ta tetrahedra,  an edgeedge with one O(1)Sr5Ta octahedra, and edges with two equivalent O(3)Sr3Ta tetrahedra. The corner-sharing octahedral tilt angles range from 11-70°. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to one Sr(2), four equivalent Sr(1), and one Ta(1) atom.