Y3Se4 crystallizes in the monoclinic C2/m space group. There are six inequivalent Y sites. In the first Y site, Y(1) is bonded to one Se(1), one Se(2), one Se(3), one Se(4), one Se(5), and one Se(6) atom to form a mixture of edge and corner-sharing YSe6 octahedra. The corner-sharing octahedral tilt angles range from 1-4°. In the second Y site, Y(2) is bonded to two equivalent Se(3) and four equivalent Se(1) atoms to form a mixture of edge and corner-sharing YSe6 octahedra. The corner-sharing octahedral tilt angles are 2°. In the third Y site, Y(3) is bonded to one Se(3), one Se(6), two equivalent Se(1), and two equivalent Se(4) atoms to form a mixture of edge and corner-sharing YSe6 octahedra. The corner-sharing octahedral tilt angles are 2°. In the fourth Y site, Y(4) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form a mixture of edge and corner-sharing YSe6 octahedra. The corner-sharing octahedral tilt angles are 2°. In the fifth Y site, Y(5) is bonded to two equivalent Se(5) and four equivalent Se(4) atoms to form edge-sharing YSe6 octahedra. In the sixth Y site, Y(6) is bonded to two equivalent Se(4), two equivalent Se(5), and two equivalent Se(6) atoms to form a mixture of edge and corner-sharing YSe6 octahedra. The corner-sharing octahedral tilt angles range from 1-4°. There are six inequivalent Se sites. In the first Se site, Se(6) is bonded to one Y(3), two equivalent Y(1), and two equivalent Y(6) atoms to form SeY5 square pyramids that share corners with three equivalent Se(3)Y6 octahedra, corners with two equivalent Se(6)Y5 square pyramids, and edges with three equivalent Se(5)Y5 square pyramids. The corner-sharing octahedral tilt angles are 4°. In the second Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Y(1), one Y(2), one Y(3), and one Y(4) atom. In the third Se site, Se(2) is bonded in a rectangular see-saw-like geometry to two equivalent Y(1) and two equivalent Y(4) atoms. In the fourth Se site, Se(3) is bonded to one Y(2), one Y(3), two equivalent Y(1), and two equivalent Y(4) atoms to form SeY6 octahedra that share corners with three equivalent Se(3)Y6 octahedra, corners with three equivalent Se(6)Y5 square pyramids, and  an edgeedge with one Se(5)Y5 square pyramid. The corner-sharing octahedra are not tilted. In the fifth Se site, Se(4) is bonded in a rectangular see-saw-like geometry to one Y(1), one Y(3), one Y(5), and one Y(6) atom. In the sixth Se site, Se(5) is bonded to one Y(5), two equivalent Y(1), and two equivalent Y(6) atoms to form SeY5 square pyramids that share corners with three equivalent Se(5)Y5 square pyramids,  an edgeedge with one Se(3)Y6 octahedra, and edges with three equivalent Se(6)Y5 square pyramids.
Y3Se4 crystallizes in the monoclinic C2/m space group. There are six inequivalent Y sites. In the first Y site, Y(1) is bonded to one Se(1), one Se(2), one Se(3), one Se(4), one Se(5), and one Se(6) atom to form a mixture of edge and corner-sharing YSe6 octahedra. The corner-sharing octahedral tilt angles range from 1-4°. The Y(1)-Se(1) bond length is 2.81 Å. The Y(1)-Se(2) bond length is 2.81 Å. The Y(1)-Se(3) bond length is 2.88 Å. The Y(1)-Se(4) bond length is 2.88 Å. The Y(1)-Se(5) bond length is 2.95 Å. The Y(1)-Se(6) bond length is 2.87 Å. In the second Y site, Y(2) is bonded to two equivalent Se(3) and four equivalent Se(1) atoms to form a mixture of edge and corner-sharing YSe6 octahedra. The corner-sharing octahedral tilt angles are 2°. Both Y(2)-Se(3) bond lengths are 2.85 Å. All Y(2)-Se(1) bond lengths are 2.86 Å. In the third Y site, Y(3) is bonded to one Se(3), one Se(6), two equivalent Se(1), and two equivalent Se(4) atoms to form a mixture of edge and corner-sharing YSe6 octahedra. The corner-sharing octahedral tilt angles are 2°. The Y(3)-Se(3) bond length is 2.85 Å. The Y(3)-Se(6) bond length is 2.90 Å. Both Y(3)-Se(1) bond lengths are 2.83 Å. Both Y(3)-Se(4) bond lengths are 2.89 Å. In the fourth Y site, Y(4) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form a mixture of edge and corner-sharing YSe6 octahedra. The corner-sharing octahedral tilt angles are 2°. Both Y(4)-Se(1) bond lengths are 2.86 Å. Both Y(4)-Se(2) bond lengths are 2.86 Å. Both Y(4)-Se(3) bond lengths are 2.88 Å. In the fifth Y site, Y(5) is bonded to two equivalent Se(5) and four equivalent Se(4) atoms to form edge-sharing YSe6 octahedra. Both Y(5)-Se(5) bond lengths are 2.86 Å. All Y(5)-Se(4) bond lengths are 2.85 Å. In the sixth Y site, Y(6) is bonded to two equivalent Se(4), two equivalent Se(5), and two equivalent Se(6) atoms to form a mixture of edge and corner-sharing YSe6 octahedra. The corner-sharing octahedral tilt angles range from 1-4°. Both Y(6)-Se(4) bond lengths are 2.83 Å. Both Y(6)-Se(5) bond lengths are 2.89 Å. Both Y(6)-Se(6) bond lengths are 2.88 Å. There are six inequivalent Se sites. In the first Se site, Se(6) is bonded to one Y(3), two equivalent Y(1), and two equivalent Y(6) atoms to form SeY5 square pyramids that share corners with three equivalent Se(3)Y6 octahedra, corners with two equivalent Se(6)Y5 square pyramids, and edges with three equivalent Se(5)Y5 square pyramids. The corner-sharing octahedral tilt angles are 4°. In the second Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Y(1), one Y(2), one Y(3), and one Y(4) atom. In the third Se site, Se(2) is bonded in a rectangular see-saw-like geometry to two equivalent Y(1) and two equivalent Y(4) atoms. In the fourth Se site, Se(3) is bonded to one Y(2), one Y(3), two equivalent Y(1), and two equivalent Y(4) atoms to form SeY6 octahedra that share corners with three equivalent Se(3)Y6 octahedra, corners with three equivalent Se(6)Y5 square pyramids, and  an edgeedge with one Se(5)Y5 square pyramid. The corner-sharing octahedra are not tilted. In the fifth Se site, Se(4) is bonded in a rectangular see-saw-like geometry to one Y(1), one Y(3), one Y(5), and one Y(6) atom. In the sixth Se site, Se(5) is bonded to one Y(5), two equivalent Y(1), and two equivalent Y(6) atoms to form SeY5 square pyramids that share corners with three equivalent Se(5)Y5 square pyramids,  an edgeedge with one Se(3)Y6 octahedra, and edges with three equivalent Se(6)Y5 square pyramids.