Ca3Ta2O8 crystallizes in the monoclinic C2/c space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 6-coordinate geometry to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms. In the second Ca site, Ca(2) is bonded in a 6-coordinate geometry to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms. Ta(1) is bonded to one O(2), one O(3), two equivalent O(1), and two equivalent O(4) atoms to form distorted edge-sharing TaO6 octahedra. There are four inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Ca(1) and two equivalent Ta(1) atoms to form distorted OCa2Ta2 tetrahedra that share corners with four equivalent O(3)Ca3Ta trigonal pyramids, edges with two equivalent O(1)Ca2Ta2 tetrahedra, and  an edgeedge with one O(3)Ca3Ta trigonal pyramid. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Ca(2), two equivalent Ca(1), and one Ta(1) atom. In the third O site, O(3) is bonded to one Ca(2), two equivalent Ca(1), and one Ta(1) atom to form distorted OCa3Ta trigonal pyramids that share corners with four equivalent O(1)Ca2Ta2 tetrahedra,  a cornercorner with one O(3)Ca3Ta trigonal pyramid,  an edgeedge with one O(1)Ca2Ta2 tetrahedra, and  an edgeedge with one O(3)Ca3Ta trigonal pyramid. In the fourth O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to one Ca(2) and two equivalent Ta(1) atoms.
Ca3Ta2O8 crystallizes in the monoclinic C2/c space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 6-coordinate geometry to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms. There is one shorter (2.24 Å) and one longer (2.53 Å) Ca(1)-O(1) bond length. There is one shorter (2.41 Å) and one longer (2.79 Å) Ca(1)-O(2) bond length. Both Ca(1)-O(3) bond lengths are 2.34 Å. In the second Ca site, Ca(2) is bonded in a 6-coordinate geometry to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms. Both Ca(2)-O(2) bond lengths are 2.36 Å. Both Ca(2)-O(3) bond lengths are 2.47 Å. Both Ca(2)-O(4) bond lengths are 2.22 Å. Ta(1) is bonded to one O(2), one O(3), two equivalent O(1), and two equivalent O(4) atoms to form distorted edge-sharing TaO6 octahedra. The Ta(1)-O(2) bond length is 1.92 Å. The Ta(1)-O(3) bond length is 1.94 Å. There is one shorter (2.05 Å) and one longer (2.13 Å) Ta(1)-O(1) bond length. Both Ta(1)-O(4) bond lengths are 2.04 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Ca(1) and two equivalent Ta(1) atoms to form distorted OCa2Ta2 tetrahedra that share corners with four equivalent O(3)Ca3Ta trigonal pyramids, edges with two equivalent O(1)Ca2Ta2 tetrahedra, and  an edgeedge with one O(3)Ca3Ta trigonal pyramid. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Ca(2), two equivalent Ca(1), and one Ta(1) atom. In the third O site, O(3) is bonded to one Ca(2), two equivalent Ca(1), and one Ta(1) atom to form distorted OCa3Ta trigonal pyramids that share corners with four equivalent O(1)Ca2Ta2 tetrahedra,  a cornercorner with one O(3)Ca3Ta trigonal pyramid,  an edgeedge with one O(1)Ca2Ta2 tetrahedra, and  an edgeedge with one O(3)Ca3Ta trigonal pyramid. In the fourth O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to one Ca(2) and two equivalent Ta(1) atoms.