Y2Se3 crystallizes in the tetragonal I-42d space group. There are three inequivalent Y sites. In the first Y site, Y(1) is bonded in a 8-coordinate geometry to two equivalent Se(2), two equivalent Se(3), and four equivalent Se(1) atoms. In the second Y site, Y(2) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(3), and four equivalent Se(2) atoms. In the third Y site, Y(3) is bonded in a 8-coordinate geometry to two equivalent Se(2), three equivalent Se(1), and three equivalent Se(3) atoms. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to one Y(2), two equivalent Y(1), and three equivalent Y(3) atoms to form distorted SeY6 octahedra that share corners with three equivalent Se(1)Y6 octahedra, corners with five equivalent Se(2)Y5 square pyramids, corners with seven equivalent Se(3)Y5 square pyramids, edges with two equivalent Se(1)Y6 octahedra,  an edgeedge with one Se(3)Y5 square pyramid, edges with three equivalent Se(2)Y5 square pyramids, faces with two equivalent Se(1)Y6 octahedra,  a faceface with one Se(2)Y5 square pyramid, and faces with two equivalent Se(3)Y5 square pyramids. The corner-sharing octahedral tilt angles range from 18-50°. In the second Se site, Se(2) is bonded to one Y(1), two equivalent Y(2), and two equivalent Y(3) atoms to form distorted SeY5 square pyramids that share corners with five equivalent Se(1)Y6 octahedra, corners with four equivalent Se(2)Y5 square pyramids, corners with four equivalent Se(3)Y5 square pyramids, edges with three equivalent Se(1)Y6 octahedra,  an edgeedge with one Se(2)Y5 square pyramid, edges with four equivalent Se(3)Y5 square pyramids,  a faceface with one Se(1)Y6 octahedra, and  a faceface with one Se(2)Y5 square pyramid. The corner-sharing octahedral tilt angles range from 23-49°. In the third Se site, Se(3) is bonded to one Y(1), one Y(2), and three equivalent Y(3) atoms to form distorted SeY5 square pyramids that share corners with seven equivalent Se(1)Y6 octahedra, corners with four equivalent Se(2)Y5 square pyramids, corners with five equivalent Se(3)Y5 square pyramids,  an edgeedge with one Se(1)Y6 octahedra, edges with four equivalent Se(2)Y5 square pyramids, faces with two equivalent Se(1)Y6 octahedra, and  a faceface with one Se(3)Y5 square pyramid. The corner-sharing octahedral tilt angles range from 15-53°.
Y2Se3 crystallizes in the tetragonal I-42d space group. There are three inequivalent Y sites. In the first Y site, Y(1) is bonded in a 8-coordinate geometry to two equivalent Se(2), two equivalent Se(3), and four equivalent Se(1) atoms. Both Y(1)-Se(2) bond lengths are 2.89 Å. Both Y(1)-Se(3) bond lengths are 3.11 Å. There are two shorter (2.89 Å) and two longer (3.14 Å) Y(1)-Se(1) bond lengths. In the second Y site, Y(2) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(3), and four equivalent Se(2) atoms. Both Y(2)-Se(1) bond lengths are 3.22 Å. Both Y(2)-Se(3) bond lengths are 2.91 Å. There are two shorter (2.90 Å) and two longer (3.10 Å) Y(2)-Se(2) bond lengths. In the third Y site, Y(3) is bonded in a 8-coordinate geometry to two equivalent Se(2), three equivalent Se(1), and three equivalent Se(3) atoms. There is one shorter (2.86 Å) and one longer (3.13 Å) Y(3)-Se(2) bond length. There are a spread of Y(3)-Se(1) bond distances ranging from 2.90-3.27 Å. There are a spread of Y(3)-Se(3) bond distances ranging from 2.88-3.10 Å. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to one Y(2), two equivalent Y(1), and three equivalent Y(3) atoms to form distorted SeY6 octahedra that share corners with three equivalent Se(1)Y6 octahedra, corners with five equivalent Se(2)Y5 square pyramids, corners with seven equivalent Se(3)Y5 square pyramids, edges with two equivalent Se(1)Y6 octahedra,  an edgeedge with one Se(3)Y5 square pyramid, edges with three equivalent Se(2)Y5 square pyramids, faces with two equivalent Se(1)Y6 octahedra,  a faceface with one Se(2)Y5 square pyramid, and faces with two equivalent Se(3)Y5 square pyramids. The corner-sharing octahedral tilt angles range from 18-50°. In the second Se site, Se(2) is bonded to one Y(1), two equivalent Y(2), and two equivalent Y(3) atoms to form distorted SeY5 square pyramids that share corners with five equivalent Se(1)Y6 octahedra, corners with four equivalent Se(2)Y5 square pyramids, corners with four equivalent Se(3)Y5 square pyramids, edges with three equivalent Se(1)Y6 octahedra,  an edgeedge with one Se(2)Y5 square pyramid, edges with four equivalent Se(3)Y5 square pyramids,  a faceface with one Se(1)Y6 octahedra, and  a faceface with one Se(2)Y5 square pyramid. The corner-sharing octahedral tilt angles range from 23-49°. In the third Se site, Se(3) is bonded to one Y(1), one Y(2), and three equivalent Y(3) atoms to form distorted SeY5 square pyramids that share corners with seven equivalent Se(1)Y6 octahedra, corners with four equivalent Se(2)Y5 square pyramids, corners with five equivalent Se(3)Y5 square pyramids,  an edgeedge with one Se(1)Y6 octahedra, edges with four equivalent Se(2)Y5 square pyramids, faces with two equivalent Se(1)Y6 octahedra, and  a faceface with one Se(3)Y5 square pyramid. The corner-sharing octahedral tilt angles range from 15-53°.