Sm3TaO7 crystallizes in the orthorhombic Pnma space group. There are two inequivalent Sm sites. In the first Sm site, Sm(1) is bonded in a 6-coordinate geometry to two equivalent O(1) and four equivalent O(4) atoms. In the second Sm site, Sm(2) is bonded to one O(3), two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted SmO7 pentagonal bipyramids that share corners with two equivalent Ta(1)O6 octahedra, corners with three equivalent Sm(2)O7 pentagonal bipyramids, edges with two equivalent Ta(1)O6 octahedra, and edges with two equivalent Sm(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 34-43°. Ta(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with four equivalent Sm(2)O7 pentagonal bipyramids, and edges with four equivalent Sm(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 34°. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Sm(1), two equivalent Sm(2), and one Ta(1) atom to form distorted OSm3Ta tetrahedra that share corners with four equivalent O(4)Sm4 tetrahedra, edges with two equivalent O(1)Sm3Ta tetrahedra, and edges with two equivalent O(4)Sm4 tetrahedra. In the second O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Sm(2) and one Ta(1) atom. In the third O site, O(3) is bonded in a distorted bent 150 degrees geometry to two equivalent Sm(2) and two equivalent Ta(1) atoms. 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 four equivalent O(1)Sm3Ta tetrahedra, corners with six equivalent O(4)Sm4 tetrahedra,  an edgeedge with one O(4)Sm4 tetrahedra, and edges with two equivalent O(1)Sm3Ta tetrahedra.
Sm3TaO7 crystallizes in the orthorhombic Pnma space group. There are two inequivalent Sm sites. In the first Sm site, Sm(1) is bonded in a 6-coordinate geometry to two equivalent O(1) and four equivalent O(4) atoms. Both Sm(1)-O(1) bond lengths are 2.39 Å. There are two shorter (2.39 Å) and two longer (2.44 Å) Sm(1)-O(4) bond lengths. In the second Sm site, Sm(2) is bonded to one O(3), two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted SmO7 pentagonal bipyramids that share corners with two equivalent Ta(1)O6 octahedra, corners with three equivalent Sm(2)O7 pentagonal bipyramids, edges with two equivalent Ta(1)O6 octahedra, and edges with two equivalent Sm(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 34-43°. The Sm(2)-O(3) bond length is 2.69 Å. There is one shorter (2.40 Å) and one longer (2.45 Å) Sm(2)-O(1) bond length. There is one shorter (2.37 Å) and one longer (2.43 Å) Sm(2)-O(2) bond length. There is one shorter (2.31 Å) and one longer (2.33 Å) Sm(2)-O(4) bond length. Ta(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form TaO6 octahedra that share corners with two equivalent Ta(1)O6 octahedra, corners with four equivalent Sm(2)O7 pentagonal bipyramids, and edges with four equivalent Sm(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 34°. Both Ta(1)-O(1) bond lengths are 2.06 Å. Both Ta(1)-O(2) bond lengths are 1.95 Å. There is one shorter (1.96 Å) and one longer (2.01 Å) Ta(1)-O(3) bond length. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Sm(1), two equivalent Sm(2), and one Ta(1) atom to form distorted OSm3Ta tetrahedra that share corners with four equivalent O(4)Sm4 tetrahedra, edges with two equivalent O(1)Sm3Ta tetrahedra, and edges with two equivalent O(4)Sm4 tetrahedra. In the second O site, O(2) is bonded in a 3-coordinate geometry to two equivalent Sm(2) and one Ta(1) atom. In the third O site, O(3) is bonded in a distorted bent 150 degrees geometry to two equivalent Sm(2) and two equivalent Ta(1) atoms. 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 four equivalent O(1)Sm3Ta tetrahedra, corners with six equivalent O(4)Sm4 tetrahedra,  an edgeedge with one O(4)Sm4 tetrahedra, and edges with two equivalent O(1)Sm3Ta tetrahedra.