Cs3FeSe3 crystallizes in the orthorhombic Cmce space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 5-coordinate geometry to two equivalent Se(2) and three equivalent Se(1) atoms. In the second Cs site, Cs(2) is bonded in a 6-coordinate geometry to one Se(1) and five equivalent Se(2) atoms. Fe(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form edge-sharing FeSe4 tetrahedra. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 7-coordinate geometry to two equivalent Cs(2), three equivalent Cs(1), and two equivalent Fe(1) atoms. In the second Se site, Se(2) is bonded in a 7-coordinate geometry to one Cs(1), five equivalent Cs(2), and one Fe(1) atom.
Cs3FeSe3 crystallizes in the orthorhombic Cmce space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 5-coordinate geometry to two equivalent Se(2) and three equivalent Se(1) atoms. Both Cs(1)-Se(2) bond lengths are 3.56 Å. There are a spread of Cs(1)-Se(1) bond distances ranging from 3.59-3.66 Å. In the second Cs site, Cs(2) is bonded in a 6-coordinate geometry to one Se(1) and five equivalent Se(2) atoms. The Cs(2)-Se(1) bond length is 3.85 Å. There are a spread of Cs(2)-Se(2) bond distances ranging from 3.66-3.89 Å. Fe(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form edge-sharing FeSe4 tetrahedra. Both Fe(1)-Se(1) bond lengths are 2.47 Å. Both Fe(1)-Se(2) bond lengths are 2.38 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 7-coordinate geometry to two equivalent Cs(2), three equivalent Cs(1), and two equivalent Fe(1) atoms. In the second Se site, Se(2) is bonded in a 7-coordinate geometry to one Cs(1), five equivalent Cs(2), and one Fe(1) atom.