[P<sub>3</sub>Se<sub>7</sub>]<sup>3−</sup>: A Phosphorus-Rich Square-Ring Selenophosphate

The new compound Cs<sub>10</sub>P<sub>8</sub>Se<sub>20</sub> features the heterotetracyclic [P<sub>3</sub>Se<sub>7</sub>]<sup>3−</sup> anion, a phosphorus-rich four-membered-ring species that possesses a P−P−P unit and formally P<sup>3+</sup> and P<sup>4+</sup> centers. It crystallizes in the orthorhombic space group <i>Pnnm</i> with <i>a</i> = 26.5456(7) Å, <i>b</i> = 8.0254(2) Å, <i>c</i> = 11.9031(4) Å, and <i>Z</i> = 2 at 100(2) K. The cyclic anion is cocrystallized with a [P<sub>2</sub>Se<sub>6</sub>]<sup>4−</sup> anion. Electronic absorption, Raman, Fourier transform infrared, and solid-state <sup>31</sup>P NMR spectroscopy studies of Cs<sub>10</sub>P<sub>8</sub>Se<sub>20</sub> are reported.