Pm2Se3 is Stibnite structured and crystallizes in the orthorhombic Pnma space group. There are two inequivalent Pm sites. In the first Pm site, Pm(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), three equivalent Se(2), and three equivalent Se(3) atoms. In the second Pm site, Pm(2) is bonded to two equivalent Se(2), two equivalent Se(3), and three equivalent Se(1) atoms to form a mixture of distorted corner and edge-sharing PmSe7 pentagonal bipyramids. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Pm(1) and three equivalent Pm(2) atoms to form a mixture of corner and edge-sharing SePm5 square pyramids. In the second Se site, Se(2) is bonded to two equivalent Pm(2) and three equivalent Pm(1) atoms to form a mixture of distorted corner and edge-sharing SePm5 trigonal bipyramids. In the third Se site, Se(3) is bonded in a 5-coordinate geometry to two equivalent Pm(2) and three equivalent Pm(1) atoms.
Pm2Se3 is Stibnite structured and crystallizes in the orthorhombic Pnma space group. There are two inequivalent Pm sites. In the first Pm site, Pm(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), three equivalent Se(2), and three equivalent Se(3) atoms. Both Pm(1)-Se(1) bond lengths are 3.05 Å. There are two shorter (3.03 Å) and one longer (3.05 Å) Pm(1)-Se(2) bond length. There are two shorter (3.03 Å) and one longer (3.52 Å) Pm(1)-Se(3) bond length. In the second Pm site, Pm(2) is bonded to two equivalent Se(2), two equivalent Se(3), and three equivalent Se(1) atoms to form a mixture of distorted corner and edge-sharing PmSe7 pentagonal bipyramids. Both Pm(2)-Se(2) bond lengths are 2.97 Å. There is one shorter (2.98 Å) and one longer (3.01 Å) Pm(2)-Se(3) bond length. There are two shorter (2.95 Å) and one longer (2.98 Å) Pm(2)-Se(1) bond length. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Pm(1) and three equivalent Pm(2) atoms to form a mixture of corner and edge-sharing SePm5 square pyramids. In the second Se site, Se(2) is bonded to two equivalent Pm(2) and three equivalent Pm(1) atoms to form a mixture of distorted corner and edge-sharing SePm5 trigonal bipyramids. In the third Se site, Se(3) is bonded in a 5-coordinate geometry to two equivalent Pm(2) and three equivalent Pm(1) atoms.