Synthesis, Characterization, and Bonding of Two New Heteropolychalcogenides: α-CsCu(S<i>_x</i>Se_4-<i>x</i>) and CsCu(S<i>_x</i>Se_6-<i>x</i>)

The Cs−Cu−Q (Q = S, Se) system has been investigated using copper metal, cesium chloride, and alkali-metal polychalcogenide salts under mild hydrothermal reaction conditions. Heteropolychalcogenide salts and mixtures of known polysulfide and polyselenide salts have been used as reagents. The reaction products contain the α-CsCuQ4 and CsCuQ6 structures. The α-CsCuQ4 phase exhibits a smooth transition in lattice parameters from the pure sulfur to the pure selenium phases, based on Vegard's law. The CsCuQ6 phase has been prepared as the pure sulfur analog and a selenium rich analog. The single-crystal structures of the disordered compounds α-CsCuS2Se2 (P212121, Z = 4, a = 5.439(1) Å, b = 8.878(2) Å, c = 13.762(4) Å) and CsCuS1.6Se4.4 (P1̄, Z = 2, a = 11.253(4) Å, b = 11.585(2) Å, c = 7.211(2) Å, α = 92.93°, β = 100.94°, γ = 74.51°) have been solved using a correlated-site occupancy model. These disordered structures display a polychalcogenide geometry in which the sulfur atoms prefer positions that are bound to copper. The optical absorption spectra of these materials have been investigated. The optical band gap varies as a function of the sulfur−selenium ratio. Extended Hückel crystal orbital calculations have been performed to investigate the electronic structure and bonding in these compounds in an attempt to explain the site distribution of sulfur and selenium.