Nd3Ti3O8Se2 crystallizes in the monoclinic P2_1/m space group. There are three inequivalent Nd sites. In the first Nd site, Nd(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(2), one O(6), and three equivalent O(4) atoms. In the second Nd site, Nd(2) is bonded in a 9-coordinate geometry to two equivalent Se(2), one O(5), one O(6), two equivalent O(8), and three equivalent O(3) atoms. In the third Nd site, Nd(3) is bonded in a 3-coordinate geometry to one Se(2), three equivalent Se(1), one O(1), and two equivalent O(2) atoms. There are three inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to one O(1), one O(5), one O(8), and three equivalent O(7) atoms to form TiO6 octahedra that share corners with two equivalent Ti(1)O6 octahedra, edges with two equivalent Ti(1)O6 octahedra, and  an edgeedge with one Ti(2)SeO5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 20°. In the second Ti site, Ti(2) is bonded to one Se(2), one O(1), one O(4), one O(8), and two equivalent O(6) atoms to form distorted TiSeO5 trigonal bipyramids that share corners with two equivalent Ti(2)SeO5 trigonal bipyramids and  an edgeedge with one Ti(1)O6 octahedra. In the third Ti site, Ti(3) is bonded in a 5-coordinate geometry to one O(2), one O(3), one O(7), one O(8), and two equivalent O(5) atoms. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Nd(1) and three equivalent Nd(3) atoms to form SeNd5 square pyramids that share corners with four equivalent Se(2)Nd5Ti pentagonal pyramids, corners with four equivalent O(4)Nd3Ti tetrahedra, edges with four equivalent Se(1)Nd5 square pyramids,  an edgeedge with one O(4)Nd3Ti tetrahedra, and  a faceface with one Se(2)Nd5Ti pentagonal pyramid. In the second Se site, Se(2) is bonded to one Nd(3), two equivalent Nd(1), two equivalent Nd(2), and one Ti(2) atom to form distorted SeNd5Ti pentagonal pyramids that share corners with four equivalent Se(1)Nd5 square pyramids, corners with four equivalent O(3)Nd3Ti tetrahedra, corners with four equivalent O(4)Nd3Ti tetrahedra, edges with two equivalent Se(2)Nd5Ti pentagonal pyramids,  an edgeedge with one O(3)Nd3Ti tetrahedra,  a faceface with one Se(1)Nd5 square pyramid, and  a faceface with one O(4)Nd3Ti tetrahedra. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Nd(3), one Ti(1), and one Ti(2) atom. In the second O site, O(2) is bonded in a trigonal planar geometry to two equivalent Nd(3) and one Ti(3) atom. In the third O site, O(3) is bonded to three equivalent Nd(2) and one Ti(3) atom to form distorted ONd3Ti tetrahedra that share corners with four equivalent Se(2)Nd5Ti pentagonal pyramids, corners with two equivalent O(3)Nd3Ti tetrahedra,  an edgeedge with one Se(2)Nd5Ti pentagonal pyramid, and edges with two equivalent O(3)Nd3Ti tetrahedra. In the fourth O site, O(4) is bonded to three equivalent Nd(1) and one Ti(2) atom to form distorted ONd3Ti tetrahedra that share corners with four equivalent Se(2)Nd5Ti pentagonal pyramids, corners with four equivalent Se(1)Nd5 square pyramids, corners with two equivalent O(4)Nd3Ti tetrahedra,  an edgeedge with one Se(1)Nd5 square pyramid, edges with two equivalent O(4)Nd3Ti tetrahedra, and  a faceface with one Se(2)Nd5Ti pentagonal pyramid. In the fifth O site, O(5) is bonded in a distorted see-saw-like geometry to one Nd(2), one Ti(1), and two equivalent Ti(3) atoms. In the sixth O site, O(6) is bonded in a distorted see-saw-like geometry to one Nd(1), one Nd(2), and two equivalent Ti(2) atoms. In the seventh O site, O(7) is bonded in a see-saw-like geometry to one Ti(3) and three equivalent Ti(1) atoms. In the eighth O site, O(8) is bonded in a 5-coordinate geometry to two equivalent Nd(2), one Ti(1), one Ti(2), and one Ti(3) atom.
Nd3Ti3O8Se2 crystallizes in the monoclinic P2_1/m space group. There are three inequivalent Nd sites. In the first Nd site, Nd(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(2), one O(6), and three equivalent O(4) atoms. Both Nd(1)-Se(1) bond lengths are 3.08 Å. Both Nd(1)-Se(2) bond lengths are 3.10 Å. The Nd(1)-O(6) bond length is 2.44 Å. There is one shorter (2.40 Å) and two longer (2.43 Å) Nd(1)-O(4) bond lengths. In the second Nd site, Nd(2) is bonded in a 9-coordinate geometry to two equivalent Se(2), one O(5), one O(6), two equivalent O(8), and three equivalent O(3) atoms. Both Nd(2)-Se(2) bond lengths are 3.05 Å. The Nd(2)-O(5) bond length is 2.52 Å. The Nd(2)-O(6) bond length is 2.63 Å. Both Nd(2)-O(8) bond lengths are 2.61 Å. There is one shorter (2.40 Å) and two longer (2.48 Å) Nd(2)-O(3) bond lengths. In the third Nd site, Nd(3) is bonded in a 3-coordinate geometry to one Se(2), three equivalent Se(1), one O(1), and two equivalent O(2) atoms. The Nd(3)-Se(2) bond length is 3.10 Å. All Nd(3)-Se(1) bond lengths are 3.03 Å. The Nd(3)-O(1) bond length is 2.45 Å. Both Nd(3)-O(2) bond lengths are 2.32 Å. There are three inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to one O(1), one O(5), one O(8), and three equivalent O(7) atoms to form TiO6 octahedra that share corners with two equivalent Ti(1)O6 octahedra, edges with two equivalent Ti(1)O6 octahedra, and  an edgeedge with one Ti(2)SeO5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 20°. The Ti(1)-O(1) bond length is 1.93 Å. The Ti(1)-O(5) bond length is 2.04 Å. The Ti(1)-O(8) bond length is 1.96 Å. There are two shorter (2.00 Å) and one longer (2.08 Å) Ti(1)-O(7) bond length. In the second Ti site, Ti(2) is bonded to one Se(2), one O(1), one O(4), one O(8), and two equivalent O(6) atoms to form distorted TiSeO5 trigonal bipyramids that share corners with two equivalent Ti(2)SeO5 trigonal bipyramids and  an edgeedge with one Ti(1)O6 octahedra. The Ti(2)-Se(2) bond length is 2.95 Å. The Ti(2)-O(1) bond length is 1.84 Å. The Ti(2)-O(4) bond length is 1.91 Å. The Ti(2)-O(8) bond length is 2.00 Å. Both Ti(2)-O(6) bond lengths are 2.01 Å. In the third Ti site, Ti(3) is bonded in a 5-coordinate geometry to one O(2), one O(3), one O(7), one O(8), and two equivalent O(5) atoms. The Ti(3)-O(2) bond length is 1.80 Å. The Ti(3)-O(3) bond length is 1.89 Å. The Ti(3)-O(7) bond length is 2.03 Å. The Ti(3)-O(8) bond length is 2.63 Å. Both Ti(3)-O(5) bond lengths are 2.01 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Nd(1) and three equivalent Nd(3) atoms to form SeNd5 square pyramids that share corners with four equivalent Se(2)Nd5Ti pentagonal pyramids, corners with four equivalent O(4)Nd3Ti tetrahedra, edges with four equivalent Se(1)Nd5 square pyramids,  an edgeedge with one O(4)Nd3Ti tetrahedra, and  a faceface with one Se(2)Nd5Ti pentagonal pyramid. In the second Se site, Se(2) is bonded to one Nd(3), two equivalent Nd(1), two equivalent Nd(2), and one Ti(2) atom to form distorted SeNd5Ti pentagonal pyramids that share corners with four equivalent Se(1)Nd5 square pyramids, corners with four equivalent O(3)Nd3Ti tetrahedra, corners with four equivalent O(4)Nd3Ti tetrahedra, edges with two equivalent Se(2)Nd5Ti pentagonal pyramids,  an edgeedge with one O(3)Nd3Ti tetrahedra,  a faceface with one Se(1)Nd5 square pyramid, and  a faceface with one O(4)Nd3Ti tetrahedra. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Nd(3), one Ti(1), and one Ti(2) atom. In the second O site, O(2) is bonded in a trigonal planar geometry to two equivalent Nd(3) and one Ti(3) atom. In the third O site, O(3) is bonded to three equivalent Nd(2) and one Ti(3) atom to form distorted ONd3Ti tetrahedra that share corners with four equivalent Se(2)Nd5Ti pentagonal pyramids, corners with two equivalent O(3)Nd3Ti tetrahedra,  an edgeedge with one Se(2)Nd5Ti pentagonal pyramid, and edges with two equivalent O(3)Nd3Ti tetrahedra. In the fourth O site, O(4) is bonded to three equivalent Nd(1) and one Ti(2) atom to form distorted ONd3Ti tetrahedra that share corners with four equivalent Se(2)Nd5Ti pentagonal pyramids, corners with four equivalent Se(1)Nd5 square pyramids, corners with two equivalent O(4)Nd3Ti tetrahedra,  an edgeedge with one Se(1)Nd5 square pyramid, edges with two equivalent O(4)Nd3Ti tetrahedra, and  a faceface with one Se(2)Nd5Ti pentagonal pyramid. In the fifth O site, O(5) is bonded in a distorted see-saw-like geometry to one Nd(2), one Ti(1), and two equivalent Ti(3) atoms. In the sixth O site, O(6) is bonded in a distorted see-saw-like geometry to one Nd(1), one Nd(2), and two equivalent Ti(2) atoms. In the seventh O site, O(7) is bonded in a see-saw-like geometry to one Ti(3) and three equivalent Ti(1) atoms. In the eighth O site, O(8) is bonded in a 5-coordinate geometry to two equivalent Nd(2), one Ti(1), one Ti(2), and one Ti(3) atom.