Ca2Ta2CuO8 crystallizes in the triclinic P-1 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 5-coordinate geometry to one O(7), two equivalent O(5), and two equivalent O(8) atoms. In the second Ca site, Ca(2) is bonded in a 5-coordinate geometry to one O(5), one O(8), two equivalent O(6), and two equivalent O(7) atoms. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(6), and one O(8) atom to form distorted edge-sharing TaO6 octahedra. In the second Ta site, Ta(2) is bonded to one O(1), one O(2), one O(4), one O(5), one O(6), and one O(7) atom to form distorted edge-sharing TaO6 octahedra. Cu(1) is bonded in a see-saw-like geometry to one O(1), one O(2), one O(3), and one O(4) atom. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(2), and one Cu(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(2), and one Cu(1) atom. In the third O site, O(3) is bonded in a distorted bent 120 degrees geometry to one Ta(1) and one Cu(1) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Ta(2) and one Cu(1) atom. In the fifth O site, O(5) is bonded in a 5-coordinate geometry to one Ca(2), two equivalent Ca(1), one Ta(1), and one Ta(2) atom. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to two equivalent Ca(2), one Ta(1), and one Ta(2) atom. In the seventh O site, O(7) is bonded to one Ca(1), two equivalent Ca(2), and one Ta(2) atom to form a mixture of distorted corner and edge-sharing OCa3Ta trigonal pyramids. In the eighth O site, O(8) is bonded to one Ca(2), two equivalent Ca(1), and one Ta(1) atom to form a mixture of distorted corner and edge-sharing OCa3Ta trigonal pyramids.
Ca2Ta2CuO8 crystallizes in the triclinic P-1 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 5-coordinate geometry to one O(7), two equivalent O(5), and two equivalent O(8) atoms. The Ca(1)-O(7) bond length is 2.27 Å. There is one shorter (2.27 Å) and one longer (2.57 Å) Ca(1)-O(5) bond length. There is one shorter (2.24 Å) and one longer (2.39 Å) Ca(1)-O(8) bond length. In the second Ca site, Ca(2) is bonded in a 5-coordinate geometry to one O(5), one O(8), two equivalent O(6), and two equivalent O(7) atoms. The Ca(2)-O(5) bond length is 2.83 Å. The Ca(2)-O(8) bond length is 2.27 Å. There is one shorter (2.27 Å) and one longer (2.56 Å) Ca(2)-O(6) bond length. There is one shorter (2.25 Å) and one longer (2.40 Å) Ca(2)-O(7) bond length. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(6), and one O(8) atom to form distorted edge-sharing TaO6 octahedra. The Ta(1)-O(1) bond length is 2.01 Å. The Ta(1)-O(2) bond length is 2.10 Å. The Ta(1)-O(3) bond length is 1.88 Å. The Ta(1)-O(5) bond length is 2.03 Å. The Ta(1)-O(6) bond length is 2.27 Å. The Ta(1)-O(8) bond length is 1.94 Å. In the second Ta site, Ta(2) is bonded to one O(1), one O(2), one O(4), one O(5), one O(6), and one O(7) atom to form distorted edge-sharing TaO6 octahedra. The Ta(2)-O(1) bond length is 2.12 Å. The Ta(2)-O(2) bond length is 1.99 Å. The Ta(2)-O(4) bond length is 1.87 Å. The Ta(2)-O(5) bond length is 2.28 Å. The Ta(2)-O(6) bond length is 2.03 Å. The Ta(2)-O(7) bond length is 1.93 Å. Cu(1) is bonded in a see-saw-like geometry to one O(1), one O(2), one O(3), and one O(4) atom. The Cu(1)-O(1) bond length is 2.10 Å. The Cu(1)-O(2) bond length is 2.20 Å. The Cu(1)-O(3) bond length is 1.89 Å. The Cu(1)-O(4) bond length is 1.88 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(2), and one Cu(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Ta(1), one Ta(2), and one Cu(1) atom. In the third O site, O(3) is bonded in a distorted bent 120 degrees geometry to one Ta(1) and one Cu(1) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Ta(2) and one Cu(1) atom. In the fifth O site, O(5) is bonded in a 5-coordinate geometry to one Ca(2), two equivalent Ca(1), one Ta(1), and one Ta(2) atom. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to two equivalent Ca(2), one Ta(1), and one Ta(2) atom. In the seventh O site, O(7) is bonded to one Ca(1), two equivalent Ca(2), and one Ta(2) atom to form a mixture of distorted corner and edge-sharing OCa3Ta trigonal pyramids. In the eighth O site, O(8) is bonded to one Ca(2), two equivalent Ca(1), and one Ta(1) atom to form a mixture of distorted corner and edge-sharing OCa3Ta trigonal pyramids.