Ionothermal Synthesis of Cu(Sb ₂ Se ₃ )[AlCl ₄ ] and Cu ₅ (Bi ₂ Se ₃ ) ₆ [AlCl ₄ ] ₅ : Monolayers of Sb ₂ Se ₃ and Bi ₂ Se ₃ Stabilized by Cu[AlCl ₄ ]

Two new layered chalcogenide–chloridoaluminate compounds, Cu(Sb 2 Se 3 )[AlCl 4 ] and Cu 5 (Bi 2 Se 3 ) 6 [AlCl 4 ] 5 , were obtained by ionothermal synthesis from Lewis‐acidic [BMIm]Cl/AlCl 3 melts at 200°C. Single‐crystal X‐ray diffraction shows that Cu(Sb 2 Se 3 )[AlCl 4 ] contains corrugated Cu(Sb 2 Se 3 )] + layers in which an intrinsically two‐dimensional Sb 2 Se 3 net built from 12‐membered rings is cross‐linked by copper(I) atoms; [AlCl 4 ] − anions separate the cationic sheets and provide a weak CuCl coordination. Cu 5 (Bi 2 Se 3 ) 6 [AlCl 4 ] 5 features terraced Cu 3 (Bi 2 Se 3 ) 6 ] 3+ layers composed of Bi 2 Se 3 ] ribbons that are connected by copper(I) atoms. The interlayer region comprises [AlCl 4 ] − anions and [CuSeCl 3 ] tetrahedra. Density functional theory‐based bonding analyses indicate highly polar AlCl and CuCl interactions, more covalent CuSe bonding, and predominantly covalent SbSe and BiSe bonds. Electronic‐structure calculations yield indirect band gaps of about 1.35 eV for Cu(Sb 2 Se 3 )[AlCl 4 ] and 0.40 eV for Cu 5 (Bi 2 Se 3 ) 6 [AlCl 4 ] 5 . Optically allowed direct transitions at slightly higher energies account for the broad absorption observed in the UV/Vis spectra. Treatment with dilute HCl partially extracts the chloridoaluminate components and promotes splitting along the layers, producing flake‐like products, albeit with substantial loss of long‐range order.