A wide band gap selenohalide Cs₉Si₈Se₂₀Cl with unprecedented [CsSe₇Cl] mixed anionic units

Chalcohalides, combining the structural and property advantages of chalcogenides and halides, have been demonstrated to be a promising system for the design of advanced photoelectric functional materials. Herein, a new chalcohalide compound, Cs₉Si₈Se₂₀Cl, has been rationally designed by introducing a nonlinear optical (NLO)-active [Si₄Se₁₀] T2-supertetrahedral unit into a halide system and synthesized experimentally by the flux method with CsCl as the flux. The compound is composed of [CsSe₇Cl] polyhedral, [CsSe₆] octahedral, and [Si₄Se₁₀] T2-supertetrahedral units. To the best of our knowledge, the [CsSe₇Cl] mixed anionic unit appears for the first time in the compound, and the rare [Si₄Se₁₀] T2-supertetrahedral unit has been demonstrated to be a potential NLO-active unit for the design of infrared NLO materials with balanced optical properties. The theoretical calculations uncovered that the strong orbital hybridization between Si-3p, Se-4p and Cl-3p orbitals results in a wide band gap of 3.075 eV in the title compound. The results enrich the structural diversity of chalcohalide compounds and provide valuable insights into the design of wide band gap selenides.