Sm2Ta2Cl2O7 crystallizes in the monoclinic C2/m space group. Sm(1) is bonded in a 9-coordinate geometry to one O(1), two equivalent O(2), three equivalent O(4), and three equivalent Cl(1) atoms. Ta(1) is bonded to one O(3), one O(4), two equivalent O(1), and two equivalent O(2) atoms to form a mixture of distorted edge and corner-sharing TaO6 octahedra. The corner-sharing octahedral tilt angles range from 0-15°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one Sm(1) and two equivalent Ta(1) atoms. In the second O site, O(2) is bonded to two equivalent Sm(1) and two equivalent Ta(1) atoms to form distorted OSm2Ta2 trigonal pyramids that share corners with five equivalent O(4)Sm3Ta tetrahedra, corners with two equivalent O(2)Sm2Ta2 trigonal pyramids,  an edgeedge with one O(2)Sm2Ta2 trigonal pyramid, and  a faceface with one O(4)Sm3Ta tetrahedra. In the third O site, O(3) is bonded in a linear geometry to two equivalent Ta(1) atoms. In the fourth O site, O(4) is bonded to three equivalent Sm(1) and one Ta(1) atom to form distorted OSm3Ta tetrahedra that share corners with two equivalent O(4)Sm3Ta tetrahedra, corners with five equivalent O(2)Sm2Ta2 trigonal pyramids, edges with two equivalent O(4)Sm3Ta tetrahedra, and  a faceface with one O(2)Sm2Ta2 trigonal pyramid. Cl(1) is bonded in a trigonal non-coplanar geometry to three equivalent Sm(1) atoms.
Sm2Ta2Cl2O7 crystallizes in the monoclinic C2/m space group. Sm(1) is bonded in a 9-coordinate geometry to one O(1), two equivalent O(2), three equivalent O(4), and three equivalent Cl(1) atoms. The Sm(1)-O(1) bond length is 2.45 Å. Both Sm(1)-O(2) bond lengths are 2.60 Å. There is one shorter (2.37 Å) and two longer (2.48 Å) Sm(1)-O(4) bond lengths. There are two shorter (2.87 Å) and one longer (2.88 Å) Sm(1)-Cl(1) bond length. Ta(1) is bonded to one O(3), one O(4), two equivalent O(1), and two equivalent O(2) atoms to form a mixture of distorted edge and corner-sharing TaO6 octahedra. The corner-sharing octahedral tilt angles range from 0-15°. The Ta(1)-O(3) bond length is 1.92 Å. The Ta(1)-O(4) bond length is 2.00 Å. Both Ta(1)-O(1) bond lengths are 1.99 Å. There is one shorter (1.96 Å) and one longer (2.26 Å) Ta(1)-O(2) bond length. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one Sm(1) and two equivalent Ta(1) atoms. In the second O site, O(2) is bonded to two equivalent Sm(1) and two equivalent Ta(1) atoms to form distorted OSm2Ta2 trigonal pyramids that share corners with five equivalent O(4)Sm3Ta tetrahedra, corners with two equivalent O(2)Sm2Ta2 trigonal pyramids,  an edgeedge with one O(2)Sm2Ta2 trigonal pyramid, and  a faceface with one O(4)Sm3Ta tetrahedra. In the third O site, O(3) is bonded in a linear geometry to two equivalent Ta(1) atoms. In the fourth O site, O(4) is bonded to three equivalent Sm(1) and one Ta(1) atom to form distorted OSm3Ta tetrahedra that share corners with two equivalent O(4)Sm3Ta tetrahedra, corners with five equivalent O(2)Sm2Ta2 trigonal pyramids, edges with two equivalent O(4)Sm3Ta tetrahedra, and  a faceface with one O(2)Sm2Ta2 trigonal pyramid. Cl(1) is bonded in a trigonal non-coplanar geometry to three equivalent Sm(1) atoms.