ErTa3O9 crystallizes in the monoclinic P2_1/m space group. Er(1) is bonded in a 8-coordinate geometry to one O(4), one O(6), two equivalent O(2), and four equivalent O(3) atoms. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(5) atoms to form corner-sharing TaO6 octahedra. The corner-sharing octahedral tilt angles range from 35-52°. In the second Ta site, Ta(2) is bonded to one O(2), one O(4), one O(5), one O(6), and two equivalent O(3) atoms to form a mixture of distorted edge and corner-sharing TaO6 octahedra. The corner-sharing octahedral tilt angles range from 34-52°. There are six inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to two equivalent Ta(1) atoms. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Er(1), one Ta(1), and one Ta(2) atom. In the third O site, O(3) is bonded to two equivalent Er(1) and two equivalent Ta(2) atoms to form a mixture of distorted edge and corner-sharing OEr2Ta2 trigonal pyramids. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Er(1) and two equivalent Ta(2) atoms. In the fifth O site, O(5) is bonded in a bent 150 degrees geometry to one Ta(1) and one Ta(2) atom. In the sixth O site, O(6) is bonded in a distorted trigonal non-coplanar geometry to one Er(1) and two equivalent Ta(2) atoms.
ErTa3O9 crystallizes in the monoclinic P2_1/m space group. Er(1) is bonded in a 8-coordinate geometry to one O(4), one O(6), two equivalent O(2), and four equivalent O(3) atoms. The Er(1)-O(4) bond length is 2.31 Å. The Er(1)-O(6) bond length is 2.29 Å. Both Er(1)-O(2) bond lengths are 2.40 Å. There are two shorter (2.31 Å) and two longer (2.49 Å) Er(1)-O(3) 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(2), and two equivalent O(5) atoms to form corner-sharing TaO6 octahedra. The corner-sharing octahedral tilt angles range from 35-52°. Both Ta(1)-O(1) bond lengths are 1.95 Å. Both Ta(1)-O(2) bond lengths are 2.03 Å. Both Ta(1)-O(5) bond lengths are 1.98 Å. In the second Ta site, Ta(2) is bonded to one O(2), one O(4), one O(5), one O(6), and two equivalent O(3) atoms to form a mixture of distorted edge and corner-sharing TaO6 octahedra. The corner-sharing octahedral tilt angles range from 34-52°. The Ta(2)-O(2) bond length is 1.95 Å. The Ta(2)-O(4) bond length is 1.98 Å. The Ta(2)-O(5) bond length is 1.84 Å. The Ta(2)-O(6) bond length is 2.00 Å. There is one shorter (2.02 Å) and one longer (2.20 Å) Ta(2)-O(3) bond length. There are six inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to two equivalent Ta(1) atoms. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Er(1), one Ta(1), and one Ta(2) atom. In the third O site, O(3) is bonded to two equivalent Er(1) and two equivalent Ta(2) atoms to form a mixture of distorted edge and corner-sharing OEr2Ta2 trigonal pyramids. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Er(1) and two equivalent Ta(2) atoms. In the fifth O site, O(5) is bonded in a bent 150 degrees geometry to one Ta(1) and one Ta(2) atom. In the sixth O site, O(6) is bonded in a distorted trigonal non-coplanar geometry to one Er(1) and two equivalent Ta(2) atoms.