Sm3TaO7 crystallizes in the orthorhombic C222_1 space group. There are two inequivalent Sm sites. In the first Sm site, Sm(1) is bonded to one O(1), one O(2), one O(4), two equivalent O(3), and two equivalent O(5) atoms to form distorted SmO7 pentagonal bipyramids that share corners with two equivalent Ta(1)O6 octahedra,  a cornercorner with one Sm(1)O7 pentagonal bipyramid, edges with two equivalent Ta(1)O6 octahedra, and edges with three equivalent Sm(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 39-49°. In the second Sm site, Sm(2) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. Ta(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(5) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with four equivalent Sm(1)O7 pentagonal bipyramids, and edges with four equivalent Sm(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 39°. There are five inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Sm(1) and two equivalent Ta(1) atoms to form distorted OSm2Ta2 trigonal pyramids that share corners with two equivalent O(4)Sm4 tetrahedra, corners with four equivalent O(5)Sm3Ta tetrahedra, corners with two equivalent O(1)Sm2Ta2 trigonal pyramids,  an edgeedge with one O(2)Sm4 tetrahedra, and edges with two equivalent O(5)Sm3Ta tetrahedra. In the second O site, O(2) is bonded to two equivalent Sm(1) and two equivalent Sm(2) atoms to form OSm4 tetrahedra that share corners with two equivalent O(2)Sm4 tetrahedra, corners with four equivalent O(5)Sm3Ta tetrahedra, corners with four equivalent O(4)Sm4 tetrahedra,  an edgeedge with one O(4)Sm4 tetrahedra, edges with two equivalent O(5)Sm3Ta tetrahedra, and  an edgeedge with one O(1)Sm2Ta2 trigonal pyramid. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Sm(2), two equivalent Sm(1), and one Ta(1) atom. In the fourth O site, O(4) is bonded to two equivalent Sm(1) and two equivalent Sm(2) atoms to form OSm4 tetrahedra that share corners with two equivalent O(4)Sm4 tetrahedra, corners with four equivalent O(5)Sm3Ta tetrahedra, corners with four equivalent O(2)Sm4 tetrahedra, corners with two equivalent O(1)Sm2Ta2 trigonal pyramids,  an edgeedge with one O(2)Sm4 tetrahedra, and edges with two equivalent O(5)Sm3Ta tetrahedra. In the fifth O site, O(5) is bonded to one Sm(2), two equivalent Sm(1), and one Ta(1) atom to form distorted OSm3Ta tetrahedra that share corners with two equivalent O(5)Sm3Ta tetrahedra, corners with two equivalent O(2)Sm4 tetrahedra, corners with two equivalent O(4)Sm4 tetrahedra, corners with two equivalent O(1)Sm2Ta2 trigonal pyramids,  an edgeedge with one O(5)Sm3Ta tetrahedra,  an edgeedge with one O(2)Sm4 tetrahedra,  an edgeedge with one O(4)Sm4 tetrahedra, and  an edgeedge with one O(1)Sm2Ta2 trigonal pyramid.
Sm3TaO7 crystallizes in the orthorhombic C222_1 space group. There are two inequivalent Sm sites. In the first Sm site, Sm(1) is bonded to one O(1), one O(2), one O(4), two equivalent O(3), and two equivalent O(5) atoms to form distorted SmO7 pentagonal bipyramids that share corners with two equivalent Ta(1)O6 octahedra,  a cornercorner with one Sm(1)O7 pentagonal bipyramid, edges with two equivalent Ta(1)O6 octahedra, and edges with three equivalent Sm(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 39-49°. The Sm(1)-O(1) bond length is 2.55 Å. The Sm(1)-O(2) bond length is 2.27 Å. The Sm(1)-O(4) bond length is 2.32 Å. There is one shorter (2.35 Å) and one longer (2.46 Å) Sm(1)-O(3) bond length. There is one shorter (2.34 Å) and one longer (2.45 Å) Sm(1)-O(5) bond length. In the second Sm site, Sm(2) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. Both Sm(2)-O(2) bond lengths are 2.41 Å. Both Sm(2)-O(3) bond lengths are 3.05 Å. Both Sm(2)-O(4) bond lengths are 2.36 Å. Both Sm(2)-O(5) bond lengths are 2.43 Å. Ta(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(5) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with four equivalent Sm(1)O7 pentagonal bipyramids, and edges with four equivalent Sm(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 39°. Both Ta(1)-O(1) bond lengths are 2.01 Å. Both Ta(1)-O(3) bond lengths are 1.94 Å. Both Ta(1)-O(5) bond lengths are 2.03 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Sm(1) and two equivalent Ta(1) atoms to form distorted OSm2Ta2 trigonal pyramids that share corners with two equivalent O(4)Sm4 tetrahedra, corners with four equivalent O(5)Sm3Ta tetrahedra, corners with two equivalent O(1)Sm2Ta2 trigonal pyramids,  an edgeedge with one O(2)Sm4 tetrahedra, and edges with two equivalent O(5)Sm3Ta tetrahedra. In the second O site, O(2) is bonded to two equivalent Sm(1) and two equivalent Sm(2) atoms to form OSm4 tetrahedra that share corners with two equivalent O(2)Sm4 tetrahedra, corners with four equivalent O(5)Sm3Ta tetrahedra, corners with four equivalent O(4)Sm4 tetrahedra,  an edgeedge with one O(4)Sm4 tetrahedra, edges with two equivalent O(5)Sm3Ta tetrahedra, and  an edgeedge with one O(1)Sm2Ta2 trigonal pyramid. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Sm(2), two equivalent Sm(1), and one Ta(1) atom. In the fourth O site, O(4) is bonded to two equivalent Sm(1) and two equivalent Sm(2) atoms to form OSm4 tetrahedra that share corners with two equivalent O(4)Sm4 tetrahedra, corners with four equivalent O(5)Sm3Ta tetrahedra, corners with four equivalent O(2)Sm4 tetrahedra, corners with two equivalent O(1)Sm2Ta2 trigonal pyramids,  an edgeedge with one O(2)Sm4 tetrahedra, and edges with two equivalent O(5)Sm3Ta tetrahedra. In the fifth O site, O(5) is bonded to one Sm(2), two equivalent Sm(1), and one Ta(1) atom to form distorted OSm3Ta tetrahedra that share corners with two equivalent O(5)Sm3Ta tetrahedra, corners with two equivalent O(2)Sm4 tetrahedra, corners with two equivalent O(4)Sm4 tetrahedra, corners with two equivalent O(1)Sm2Ta2 trigonal pyramids,  an edgeedge with one O(5)Sm3Ta tetrahedra,  an edgeedge with one O(2)Sm4 tetrahedra,  an edgeedge with one O(4)Sm4 tetrahedra, and  an edgeedge with one O(1)Sm2Ta2 trigonal pyramid.