Cs3TaO8 crystallizes in the tetragonal I-42m space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form CsO8 hexagonal bipyramids that share corners with four equivalent Ta(1)O8 hexagonal bipyramids, corners with four equivalent O(1)Cs3TaO trigonal bipyramids, and edges with two equivalent Ta(1)O8 hexagonal bipyramids. In the second Cs site, Cs(2) is bonded in a 12-coordinate geometry to four equivalent O(1) and eight equivalent O(2) atoms. Ta(1) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form distorted TaO8 hexagonal bipyramids that share corners with four equivalent Cs(1)O8 hexagonal bipyramids, corners with four equivalent O(1)Cs3TaO trigonal bipyramids, and edges with two equivalent Cs(1)O8 hexagonal bipyramids. There are two inequivalent O sites. In the first O site, O(1) is bonded to one Cs(1), two equivalent Cs(2), one Ta(1), and one O(2) atom to form distorted OCs3TaO trigonal bipyramids that share  a cornercorner with one Cs(1)O8 hexagonal bipyramid,  a cornercorner with one Ta(1)O8 hexagonal bipyramid, corners with ten equivalent O(1)Cs3TaO trigonal bipyramids, and  an edgeedge with one O(1)Cs3TaO trigonal bipyramid. In the second O site, O(2) is bonded in a 1-coordinate geometry to one Cs(1), four equivalent Cs(2), one Ta(1), and one O(1) atom.
Cs3TaO8 crystallizes in the tetragonal I-42m space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form CsO8 hexagonal bipyramids that share corners with four equivalent Ta(1)O8 hexagonal bipyramids, corners with four equivalent O(1)Cs3TaO trigonal bipyramids, and edges with two equivalent Ta(1)O8 hexagonal bipyramids. All Cs(1)-O(1) bond lengths are 3.00 Å. All Cs(1)-O(2) bond lengths are 3.20 Å. In the second Cs site, Cs(2) is bonded in a 12-coordinate geometry to four equivalent O(1) and eight equivalent O(2) atoms. All Cs(2)-O(1) bond lengths are 2.98 Å. There are four shorter (3.31 Å) and four longer (3.49 Å) Cs(2)-O(2) bond lengths. Ta(1) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form distorted TaO8 hexagonal bipyramids that share corners with four equivalent Cs(1)O8 hexagonal bipyramids, corners with four equivalent O(1)Cs3TaO trigonal bipyramids, and edges with two equivalent Cs(1)O8 hexagonal bipyramids. All Ta(1)-O(1) bond lengths are 2.01 Å. All Ta(1)-O(2) bond lengths are 2.07 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded to one Cs(1), two equivalent Cs(2), one Ta(1), and one O(2) atom to form distorted OCs3TaO trigonal bipyramids that share  a cornercorner with one Cs(1)O8 hexagonal bipyramid,  a cornercorner with one Ta(1)O8 hexagonal bipyramid, corners with ten equivalent O(1)Cs3TaO trigonal bipyramids, and  an edgeedge with one O(1)Cs3TaO trigonal bipyramid. The O(1)-O(2) bond length is 1.51 Å. In the second O site, O(2) is bonded in a 1-coordinate geometry to one Cs(1), four equivalent Cs(2), one Ta(1), and one O(1) atom.