Cs4P2Se9 crystallizes in the monoclinic C2/c space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 8-coordinate geometry to one Se(2), one Se(3), two equivalent Se(1), and four equivalent Se(4) atoms. In the second Cs site, Cs(2) is bonded in a 8-coordinate geometry to one Se(3), one Se(4), one Se(5), two equivalent Se(1), and three equivalent Se(2) atoms. P(1) is bonded in a tetrahedral geometry to one Se(1), one Se(2), one Se(3), and one Se(4) atom. There are five inequivalent Se sites. In the first Se site, Se(4) is bonded in a 6-coordinate geometry to one Cs(2), four equivalent Cs(1), and one P(1) atom. In the second Se site, Se(5) is bonded in a distorted see-saw-like geometry to two equivalent Cs(2) and two equivalent Se(3) atoms. In the third Se site, Se(1) is bonded in a 5-coordinate geometry to two equivalent Cs(1), two equivalent Cs(2), and one P(1) atom. In the fourth Se site, Se(2) is bonded to one Cs(1), three equivalent Cs(2), and one P(1) atom to form a mixture of distorted edge and corner-sharing SeCs4P square pyramids. In the fifth Se site, Se(3) is bonded in a distorted single-bond geometry to one Cs(1), one Cs(2), one P(1), and one Se(5) atom.
Cs4P2Se9 crystallizes in the monoclinic C2/c space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 8-coordinate geometry to one Se(2), one Se(3), two equivalent Se(1), and four equivalent Se(4) atoms. The Cs(1)-Se(2) bond length is 3.67 Å. The Cs(1)-Se(3) bond length is 3.86 Å. There is one shorter (3.68 Å) and one longer (3.85 Å) Cs(1)-Se(1) bond length. There are a spread of Cs(1)-Se(4) bond distances ranging from 3.75-3.87 Å. In the second Cs site, Cs(2) is bonded in a 8-coordinate geometry to one Se(3), one Se(4), one Se(5), two equivalent Se(1), and three equivalent Se(2) atoms. The Cs(2)-Se(3) bond length is 4.04 Å. The Cs(2)-Se(4) bond length is 3.88 Å. The Cs(2)-Se(5) bond length is 3.88 Å. There is one shorter (3.81 Å) and one longer (3.84 Å) Cs(2)-Se(1) bond length. There are a spread of Cs(2)-Se(2) bond distances ranging from 3.73-3.79 Å. P(1) is bonded in a tetrahedral geometry to one Se(1), one Se(2), one Se(3), and one Se(4) atom. The P(1)-Se(1) bond length is 2.16 Å. The P(1)-Se(2) bond length is 2.16 Å. The P(1)-Se(3) bond length is 2.36 Å. The P(1)-Se(4) bond length is 2.19 Å. There are five inequivalent Se sites. In the first Se site, Se(4) is bonded in a 6-coordinate geometry to one Cs(2), four equivalent Cs(1), and one P(1) atom. In the second Se site, Se(5) is bonded in a distorted see-saw-like geometry to two equivalent Cs(2) and two equivalent Se(3) atoms. Both Se(5)-Se(3) bond lengths are 2.35 Å. In the third Se site, Se(1) is bonded in a 5-coordinate geometry to two equivalent Cs(1), two equivalent Cs(2), and one P(1) atom. In the fourth Se site, Se(2) is bonded to one Cs(1), three equivalent Cs(2), and one P(1) atom to form a mixture of distorted edge and corner-sharing SeCs4P square pyramids. In the fifth Se site, Se(3) is bonded in a distorted single-bond geometry to one Cs(1), one Cs(2), one P(1), and one Se(5) atom.