Reactive flux syntheses, crystal structures and band gaps of AInS2 (A = Rb, Cs)

High-quality single crystals of RbInS2, CsInS2 were isolated from the halide flux as a major phase of the repeated fusion of CaS and In2S3 followed by slow cooling. RbInS2 and CsInS2 are practically isostructural with KInS2. RbInS2: C2/c, a = 11.071(6) Å, b = 11.068(1) Å, c = 15.610(7) Å, β = 100.36(3)°, V = 1882(2) Å3, Z = 16, dx = 3.74 g/cm3, R = 6.38%, Rw = 6.42%. CsInS2: C2/c, a = 11.197(3) Å, b = 11.158(3) Å, c = 16.358(4) Å, β = 99.92(2)°, V = 2013(2) Å3, Z = 16, dx = 4.12 g/cm3, R = 5.60%, Rw = 6.20%. The AInS2 (A = Rb, Cs) structure is characterized by double layers of vertex-sharing [In4S10] units that each consists of four [InS4] polyhedra. The charge-balancing alkali-metal cations are stuffed into the channels created by the packing of these anionic [In4S10] blocks. The optical reflectance measurements show a band gap of 3.3 eV for RbInS2 and 3.4 eV for CsInS2, suggesting that both are semiconductors.