Na2Ta4O11 crystallizes in the trigonal R-3c space group. Na(1) is bonded in a 7-coordinate geometry to one O(3) and six equivalent O(1) atoms. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to two equivalent O(1), two equivalent O(3), and three equivalent O(2) atoms to form distorted TaO7 pentagonal bipyramids that share corners with two equivalent Ta(2)O6 octahedra, corners with two equivalent Ta(1)O7 pentagonal bipyramids, and edges with four equivalent Ta(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 45°. In the second Ta site, Ta(2) is bonded to six equivalent O(1) atoms to form corner-sharing TaO6 octahedra. There are three inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Na(1), one Ta(1), and one Ta(2) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to three equivalent Ta(1) atoms. In the third O site, O(3) is bonded to one Na(1) and three equivalent Ta(1) atoms to form distorted corner-sharing ONaTa3 tetrahedra.
Na2Ta4O11 crystallizes in the trigonal R-3c space group. Na(1) is bonded in a 7-coordinate geometry to one O(3) and six equivalent O(1) atoms. The Na(1)-O(3) bond length is 2.62 Å. There are three shorter (2.45 Å) and three longer (2.72 Å) Na(1)-O(1) bond lengths. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to two equivalent O(1), two equivalent O(3), and three equivalent O(2) atoms to form distorted TaO7 pentagonal bipyramids that share corners with two equivalent Ta(2)O6 octahedra, corners with two equivalent Ta(1)O7 pentagonal bipyramids, and edges with four equivalent Ta(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 45°. Both Ta(1)-O(1) bond lengths are 1.97 Å. Both Ta(1)-O(3) bond lengths are 2.02 Å. There are two shorter (2.00 Å) and one longer (2.42 Å) Ta(1)-O(2) bond length. In the second Ta site, Ta(2) is bonded to six equivalent O(1) atoms to form corner-sharing TaO6 octahedra. All Ta(2)-O(1) bond lengths are 1.99 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Na(1), one Ta(1), and one Ta(2) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to three equivalent Ta(1) atoms. In the third O site, O(3) is bonded to one Na(1) and three equivalent Ta(1) atoms to form distorted corner-sharing ONaTa3 tetrahedra.