Sm2O3 crystallizes in the monoclinic C2/m space group. There are three inequivalent Sm sites. In the first Sm site, Sm(1) is bonded in a 6-coordinate geometry to one O(4), one O(5), two equivalent O(1), and two equivalent O(2) atoms. In the second Sm site, Sm(2) is bonded in a 7-coordinate geometry to one O(1), one O(3), two equivalent O(5), and three equivalent O(4) atoms. In the third Sm site, Sm(3) is bonded in a 7-coordinate geometry to two equivalent O(2), two equivalent O(5), and three equivalent O(3) atoms. There are five inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Sm(2) and four equivalent Sm(1) atoms to form OSm6 octahedra that share corners with two equivalent O(4)Sm4 tetrahedra, corners with four equivalent O(2)Sm4 tetrahedra, edges with two equivalent O(1)Sm6 octahedra, edges with four equivalent O(5)Sm5 square pyramids, edges with two equivalent O(2)Sm4 tetrahedra, and edges with four equivalent O(4)Sm4 tetrahedra. In the second O site, O(2) is bonded to two equivalent Sm(1) and two equivalent Sm(3) atoms to form OSm4 tetrahedra that share corners with two equivalent O(1)Sm6 octahedra, corners with two equivalent O(5)Sm5 square pyramids, corners with two equivalent O(2)Sm4 tetrahedra, corners with two equivalent O(4)Sm4 tetrahedra,  an edgeedge with one O(1)Sm6 octahedra, edges with two equivalent O(5)Sm5 square pyramids, and  an edgeedge with one O(2)Sm4 tetrahedra. The corner-sharing octahedral tilt angles are 15°. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Sm(2) and three equivalent Sm(3) atoms. In the fourth O site, O(4) is bonded to one Sm(1) and three equivalent Sm(2) atoms to form OSm4 tetrahedra that share  a cornercorner with one O(1)Sm6 octahedra, corners with five equivalent O(5)Sm5 square pyramids, corners with two equivalent O(2)Sm4 tetrahedra, corners with two equivalent O(4)Sm4 tetrahedra, edges with two equivalent O(1)Sm6 octahedra,  an edgeedge with one O(5)Sm5 square pyramid, and edges with two equivalent O(4)Sm4 tetrahedra. The corner-sharing octahedral tilt angles are 50°. In the fifth O site, O(5) is bonded to one Sm(1), two equivalent Sm(2), and two equivalent Sm(3) atoms to form distorted OSm5 square pyramids that share corners with two equivalent O(2)Sm4 tetrahedra, corners with five equivalent O(4)Sm4 tetrahedra, edges with two equivalent O(1)Sm6 octahedra, edges with two equivalent O(5)Sm5 square pyramids,  an edgeedge with one O(4)Sm4 tetrahedra, and edges with two equivalent O(2)Sm4 tetrahedra.
Sm2O3 crystallizes in the monoclinic C2/m space group. There are three inequivalent Sm sites. In the first Sm site, Sm(1) is bonded in a 6-coordinate geometry to one O(4), one O(5), two equivalent O(1), and two equivalent O(2) atoms. The Sm(1)-O(4) bond length is 2.26 Å. The Sm(1)-O(5) bond length is 2.33 Å. Both Sm(1)-O(1) bond lengths are 2.56 Å. Both Sm(1)-O(2) bond lengths are 2.27 Å. In the second Sm site, Sm(2) is bonded in a 7-coordinate geometry to one O(1), one O(3), two equivalent O(5), and three equivalent O(4) atoms. The Sm(2)-O(1) bond length is 2.77 Å. The Sm(2)-O(3) bond length is 2.36 Å. Both Sm(2)-O(5) bond lengths are 2.47 Å. There is one shorter (2.29 Å) and two longer (2.31 Å) Sm(2)-O(4) bond lengths. In the third Sm site, Sm(3) is bonded in a 7-coordinate geometry to two equivalent O(2), two equivalent O(5), and three equivalent O(3) atoms. There is one shorter (2.31 Å) and one longer (2.43 Å) Sm(3)-O(2) bond length. Both Sm(3)-O(5) bond lengths are 2.54 Å. There are two shorter (2.30 Å) and one longer (2.67 Å) Sm(3)-O(3) bond length. There are five inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Sm(2) and four equivalent Sm(1) atoms to form OSm6 octahedra that share corners with two equivalent O(4)Sm4 tetrahedra, corners with four equivalent O(2)Sm4 tetrahedra, edges with two equivalent O(1)Sm6 octahedra, edges with four equivalent O(5)Sm5 square pyramids, edges with two equivalent O(2)Sm4 tetrahedra, and edges with four equivalent O(4)Sm4 tetrahedra. In the second O site, O(2) is bonded to two equivalent Sm(1) and two equivalent Sm(3) atoms to form OSm4 tetrahedra that share corners with two equivalent O(1)Sm6 octahedra, corners with two equivalent O(5)Sm5 square pyramids, corners with two equivalent O(2)Sm4 tetrahedra, corners with two equivalent O(4)Sm4 tetrahedra,  an edgeedge with one O(1)Sm6 octahedra, edges with two equivalent O(5)Sm5 square pyramids, and  an edgeedge with one O(2)Sm4 tetrahedra. The corner-sharing octahedral tilt angles are 15°. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Sm(2) and three equivalent Sm(3) atoms. In the fourth O site, O(4) is bonded to one Sm(1) and three equivalent Sm(2) atoms to form OSm4 tetrahedra that share  a cornercorner with one O(1)Sm6 octahedra, corners with five equivalent O(5)Sm5 square pyramids, corners with two equivalent O(2)Sm4 tetrahedra, corners with two equivalent O(4)Sm4 tetrahedra, edges with two equivalent O(1)Sm6 octahedra,  an edgeedge with one O(5)Sm5 square pyramid, and edges with two equivalent O(4)Sm4 tetrahedra. The corner-sharing octahedral tilt angles are 50°. In the fifth O site, O(5) is bonded to one Sm(1), two equivalent Sm(2), and two equivalent Sm(3) atoms to form distorted OSm5 square pyramids that share corners with two equivalent O(2)Sm4 tetrahedra, corners with five equivalent O(4)Sm4 tetrahedra, edges with two equivalent O(1)Sm6 octahedra, edges with two equivalent O(5)Sm5 square pyramids,  an edgeedge with one O(4)Sm4 tetrahedra, and edges with two equivalent O(2)Sm4 tetrahedra.