Tb3NSe2O crystallizes in the monoclinic C2/m space group. There are three inequivalent Tb sites. In the first Tb site, Tb(1) is bonded in a 7-coordinate geometry to two equivalent N(1), three equivalent Se(1), and two equivalent O(1) atoms. In the second Tb site, Tb(2) is bonded in a 6-coordinate geometry to one N(1), one Se(1), two equivalent Se(2), and two equivalent O(1) atoms. In the third Tb site, Tb(3) is bonded to one N(1), two equivalent Se(1), and three equivalent Se(2) atoms to form distorted edge-sharing TbSe5N octahedra. N(1) is bonded to one Tb(2), one Tb(3), and two equivalent Tb(1) atoms to form NTb4 tetrahedra that share corners with three equivalent Se(1)Tb6 octahedra, corners with five equivalent Se(2)Tb5 square pyramids, corners with two equivalent N(1)Tb4 tetrahedra, corners with two equivalent O(1)Tb4 tetrahedra, edges with three equivalent Se(1)Tb6 octahedra, and edges with two equivalent O(1)Tb4 tetrahedra. The corner-sharing octahedral tilt angles range from 28-40°. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Tb(2), two equivalent Tb(3), and three equivalent Tb(1) atoms to form distorted SeTb6 octahedra that share corners with two equivalent Se(2)Tb5 square pyramids, corners with three equivalent N(1)Tb4 tetrahedra, corners with four equivalent O(1)Tb4 tetrahedra, edges with four equivalent Se(1)Tb6 octahedra, edges with three equivalent Se(2)Tb5 square pyramids, edges with two equivalent O(1)Tb4 tetrahedra, and edges with three equivalent N(1)Tb4 tetrahedra. In the second Se site, Se(2) is bonded to two equivalent Tb(2) and three equivalent Tb(3) atoms to form SeTb5 square pyramids that share corners with two equivalent Se(1)Tb6 octahedra, corners with two equivalent O(1)Tb4 tetrahedra, corners with five equivalent N(1)Tb4 tetrahedra, edges with three equivalent Se(1)Tb6 octahedra, edges with four equivalent Se(2)Tb5 square pyramids, and  an edgeedge with one O(1)Tb4 tetrahedra. The corner-sharing octahedral tilt angles are 9°. O(1) is bonded to two equivalent Tb(1) and two equivalent Tb(2) atoms to form OTb4 tetrahedra that share corners with four equivalent Se(1)Tb6 octahedra, corners with two equivalent Se(2)Tb5 square pyramids, corners with two equivalent N(1)Tb4 tetrahedra, corners with two equivalent O(1)Tb4 tetrahedra, edges with two equivalent Se(1)Tb6 octahedra,  an edgeedge with one Se(2)Tb5 square pyramid,  an edgeedge with one O(1)Tb4 tetrahedra, and edges with two equivalent N(1)Tb4 tetrahedra. The corner-sharing octahedral tilt angles range from 36-50°.
Tb3NSe2O crystallizes in the monoclinic C2/m space group. There are three inequivalent Tb sites. In the first Tb site, Tb(1) is bonded in a 7-coordinate geometry to two equivalent N(1), three equivalent Se(1), and two equivalent O(1) atoms. Both Tb(1)-N(1) bond lengths are 2.35 Å. There is one shorter (3.06 Å) and two longer (3.10 Å) Tb(1)-Se(1) bond lengths. There is one shorter (2.30 Å) and one longer (2.33 Å) Tb(1)-O(1) bond length. In the second Tb site, Tb(2) is bonded in a 6-coordinate geometry to one N(1), one Se(1), two equivalent Se(2), and two equivalent O(1) atoms. The Tb(2)-N(1) bond length is 2.24 Å. The Tb(2)-Se(1) bond length is 2.96 Å. Both Tb(2)-Se(2) bond lengths are 3.04 Å. Both Tb(2)-O(1) bond lengths are 2.29 Å. In the third Tb site, Tb(3) is bonded to one N(1), two equivalent Se(1), and three equivalent Se(2) atoms to form distorted edge-sharing TbSe5N octahedra. The Tb(3)-N(1) bond length is 2.18 Å. Both Tb(3)-Se(1) bond lengths are 2.86 Å. There are two shorter (2.89 Å) and one longer (3.23 Å) Tb(3)-Se(2) bond length. N(1) is bonded to one Tb(2), one Tb(3), and two equivalent Tb(1) atoms to form NTb4 tetrahedra that share corners with three equivalent Se(1)Tb6 octahedra, corners with five equivalent Se(2)Tb5 square pyramids, corners with two equivalent N(1)Tb4 tetrahedra, corners with two equivalent O(1)Tb4 tetrahedra, edges with three equivalent Se(1)Tb6 octahedra, and edges with two equivalent O(1)Tb4 tetrahedra. The corner-sharing octahedral tilt angles range from 28-40°. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Tb(2), two equivalent Tb(3), and three equivalent Tb(1) atoms to form distorted SeTb6 octahedra that share corners with two equivalent Se(2)Tb5 square pyramids, corners with three equivalent N(1)Tb4 tetrahedra, corners with four equivalent O(1)Tb4 tetrahedra, edges with four equivalent Se(1)Tb6 octahedra, edges with three equivalent Se(2)Tb5 square pyramids, edges with two equivalent O(1)Tb4 tetrahedra, and edges with three equivalent N(1)Tb4 tetrahedra. In the second Se site, Se(2) is bonded to two equivalent Tb(2) and three equivalent Tb(3) atoms to form SeTb5 square pyramids that share corners with two equivalent Se(1)Tb6 octahedra, corners with two equivalent O(1)Tb4 tetrahedra, corners with five equivalent N(1)Tb4 tetrahedra, edges with three equivalent Se(1)Tb6 octahedra, edges with four equivalent Se(2)Tb5 square pyramids, and  an edgeedge with one O(1)Tb4 tetrahedra. The corner-sharing octahedral tilt angles are 9°. O(1) is bonded to two equivalent Tb(1) and two equivalent Tb(2) atoms to form OTb4 tetrahedra that share corners with four equivalent Se(1)Tb6 octahedra, corners with two equivalent Se(2)Tb5 square pyramids, corners with two equivalent N(1)Tb4 tetrahedra, corners with two equivalent O(1)Tb4 tetrahedra, edges with two equivalent Se(1)Tb6 octahedra,  an edgeedge with one Se(2)Tb5 square pyramid,  an edgeedge with one O(1)Tb4 tetrahedra, and edges with two equivalent N(1)Tb4 tetrahedra. The corner-sharing octahedral tilt angles range from 36-50°.