ErTa7O19 crystallizes in the hexagonal P-6c2 space group. Er(1) is bonded in a 8-coordinate geometry to two equivalent O(3) and six equivalent O(4) atoms. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded in a 7-coordinate geometry to one O(2), one O(3), one O(4), one O(5), and three equivalent O(1) atoms. In the second Ta site, Ta(2) is bonded in an octahedral geometry to six equivalent O(4) atoms. There are five inequivalent O sites. In the first O site, O(4) is bonded in a distorted trigonal planar geometry to one Er(1), one Ta(1), and one Ta(2) atom. In the second O site, O(5) is bonded in a trigonal planar geometry to three equivalent Ta(1) atoms. In the third O site, O(1) is bonded in a distorted bent 150 degrees geometry to three equivalent Ta(1) atoms. In the fourth O site, O(2) is bonded in a bent 150 degrees geometry to two equivalent Ta(1) atoms. In the fifth O site, O(3) is bonded to one Er(1) and three equivalent Ta(1) atoms to form distorted corner-sharing OErTa3 tetrahedra.
ErTa7O19 crystallizes in the hexagonal P-6c2 space group. Er(1) is bonded in a 8-coordinate geometry to two equivalent O(3) and six equivalent O(4) atoms. Both Er(1)-O(3) bond lengths are 2.62 Å. All Er(1)-O(4) bond lengths are 2.39 Å. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded in a 7-coordinate geometry to one O(2), one O(3), one O(4), one O(5), and three equivalent O(1) atoms. The Ta(1)-O(2) bond length is 1.93 Å. The Ta(1)-O(3) bond length is 2.07 Å. The Ta(1)-O(4) bond length is 2.04 Å. The Ta(1)-O(5) bond length is 2.02 Å. There are two shorter (2.00 Å) and one longer (2.45 Å) Ta(1)-O(1) bond length. In the second Ta site, Ta(2) is bonded in an octahedral geometry to six equivalent O(4) atoms. All Ta(2)-O(4) bond lengths are 2.01 Å. There are five inequivalent O sites. In the first O site, O(4) is bonded in a distorted trigonal planar geometry to one Er(1), one Ta(1), and one Ta(2) atom. In the second O site, O(5) is bonded in a trigonal planar geometry to three equivalent Ta(1) atoms. In the third O site, O(1) is bonded in a distorted bent 150 degrees geometry to three equivalent Ta(1) atoms. In the fourth O site, O(2) is bonded in a bent 150 degrees geometry to two equivalent Ta(1) atoms. In the fifth O site, O(3) is bonded to one Er(1) and three equivalent Ta(1) atoms to form distorted corner-sharing OErTa3 tetrahedra.