Nb2Se3 crystallizes in the monoclinic Pm space group. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one Se(2), one Se(3), one Se(4), and two equivalent Se(6) atoms to form distorted NbSe5 trigonal bipyramids that share corners with four equivalent Nb(4)Se6 octahedra, corners with two equivalent Nb(2)Se6 pentagonal pyramids, corners with two equivalent Nb(1)Se5 trigonal bipyramids, and edges with two equivalent Nb(2)Se6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 40-43°. In the second Nb site, Nb(2) is bonded to one Se(5), one Se(6), two equivalent Se(2), and two equivalent Se(3) atoms to form distorted NbSe6 pentagonal pyramids that share corners with two equivalent Nb(4)Se6 octahedra, corners with two equivalent Nb(1)Se5 trigonal bipyramids, edges with two equivalent Nb(2)Se6 pentagonal pyramids, edges with two equivalent Nb(1)Se5 trigonal bipyramids, and  a faceface with one Nb(4)Se6 octahedra. The corner-sharing octahedral tilt angles are 46°. In the third Nb site, Nb(3) is bonded in a 7-coordinate geometry to one Se(2), two equivalent Se(1), two equivalent Se(5), and two equivalent Se(6) atoms. In the fourth Nb site, Nb(4) is bonded to one Se(1), one Se(5), two equivalent Se(2), and two equivalent Se(4) atoms to form NbSe6 octahedra that share corners with two equivalent Nb(2)Se6 pentagonal pyramids, corners with four equivalent Nb(1)Se5 trigonal bipyramids, edges with two equivalent Nb(4)Se6 octahedra, and  a faceface with one Nb(2)Se6 pentagonal pyramid. There are six inequivalent Se sites. In the first Se site, Se(1) is bonded in a 3-coordinate geometry to one Nb(4) and two equivalent Nb(3) atoms. In the second Se site, Se(2) is bonded to one Nb(1), one Nb(3), two equivalent Nb(2), and two equivalent Nb(4) atoms to form distorted SeNb6 pentagonal pyramids that share corners with two equivalent Se(6)Nb5 trigonal bipyramids, edges with two equivalent Se(2)Nb6 pentagonal pyramids, and edges with two equivalent Se(6)Nb5 trigonal bipyramids. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Nb(1) and two equivalent Nb(2) atoms. In the fourth Se site, Se(4) is bonded in a 3-coordinate geometry to one Nb(1) and two equivalent Nb(4) atoms. In the fifth Se site, Se(5) is bonded in a distorted rectangular see-saw-like geometry to one Nb(2), one Nb(4), and two equivalent Nb(3) atoms. In the sixth Se site, Se(6) is bonded to one Nb(2), two equivalent Nb(1), and two equivalent Nb(3) atoms to form distorted SeNb5 trigonal bipyramids that share corners with two equivalent Se(2)Nb6 pentagonal pyramids, edges with two equivalent Se(2)Nb6 pentagonal pyramids, and edges with two equivalent Se(6)Nb5 trigonal bipyramids.
Nb2Se3 crystallizes in the monoclinic Pm space group. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one Se(2), one Se(3), one Se(4), and two equivalent Se(6) atoms to form distorted NbSe5 trigonal bipyramids that share corners with four equivalent Nb(4)Se6 octahedra, corners with two equivalent Nb(2)Se6 pentagonal pyramids, corners with two equivalent Nb(1)Se5 trigonal bipyramids, and edges with two equivalent Nb(2)Se6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 40-43°. The Nb(1)-Se(2) bond length is 2.65 Å. The Nb(1)-Se(3) bond length is 2.60 Å. The Nb(1)-Se(4) bond length is 2.48 Å. Both Nb(1)-Se(6) bond lengths are 2.73 Å. In the second Nb site, Nb(2) is bonded to one Se(5), one Se(6), two equivalent Se(2), and two equivalent Se(3) atoms to form distorted NbSe6 pentagonal pyramids that share corners with two equivalent Nb(4)Se6 octahedra, corners with two equivalent Nb(1)Se5 trigonal bipyramids, edges with two equivalent Nb(2)Se6 pentagonal pyramids, edges with two equivalent Nb(1)Se5 trigonal bipyramids, and  a faceface with one Nb(4)Se6 octahedra. The corner-sharing octahedral tilt angles are 46°. The Nb(2)-Se(5) bond length is 2.69 Å. The Nb(2)-Se(6) bond length is 2.67 Å. Both Nb(2)-Se(2) bond lengths are 2.60 Å. Both Nb(2)-Se(3) bond lengths are 2.59 Å. In the third Nb site, Nb(3) is bonded in a 7-coordinate geometry to one Se(2), two equivalent Se(1), two equivalent Se(5), and two equivalent Se(6) atoms. The Nb(3)-Se(2) bond length is 2.77 Å. Both Nb(3)-Se(1) bond lengths are 2.82 Å. Both Nb(3)-Se(5) bond lengths are 2.61 Å. Both Nb(3)-Se(6) bond lengths are 2.74 Å. In the fourth Nb site, Nb(4) is bonded to one Se(1), one Se(5), two equivalent Se(2), and two equivalent Se(4) atoms to form NbSe6 octahedra that share corners with two equivalent Nb(2)Se6 pentagonal pyramids, corners with four equivalent Nb(1)Se5 trigonal bipyramids, edges with two equivalent Nb(4)Se6 octahedra, and  a faceface with one Nb(2)Se6 pentagonal pyramid. The Nb(4)-Se(1) bond length is 2.45 Å. The Nb(4)-Se(5) bond length is 2.63 Å. Both Nb(4)-Se(2) bond lengths are 2.66 Å. Both Nb(4)-Se(4) bond lengths are 2.62 Å. There are six inequivalent Se sites. In the first Se site, Se(1) is bonded in a 3-coordinate geometry to one Nb(4) and two equivalent Nb(3) atoms. In the second Se site, Se(2) is bonded to one Nb(1), one Nb(3), two equivalent Nb(2), and two equivalent Nb(4) atoms to form distorted SeNb6 pentagonal pyramids that share corners with two equivalent Se(6)Nb5 trigonal bipyramids, edges with two equivalent Se(2)Nb6 pentagonal pyramids, and edges with two equivalent Se(6)Nb5 trigonal bipyramids. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Nb(1) and two equivalent Nb(2) atoms. In the fourth Se site, Se(4) is bonded in a 3-coordinate geometry to one Nb(1) and two equivalent Nb(4) atoms. In the fifth Se site, Se(5) is bonded in a distorted rectangular see-saw-like geometry to one Nb(2), one Nb(4), and two equivalent Nb(3) atoms. In the sixth Se site, Se(6) is bonded to one Nb(2), two equivalent Nb(1), and two equivalent Nb(3) atoms to form distorted SeNb5 trigonal bipyramids that share corners with two equivalent Se(2)Nb6 pentagonal pyramids, edges with two equivalent Se(2)Nb6 pentagonal pyramids, and edges with two equivalent Se(6)Nb5 trigonal bipyramids.