Direct Gap Semiconductors Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5

New quaternary thioiodides Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5 have been synthesized by isothermal heating as well as chemical vapor transport. Pb2BiS2I3 and Sn2BiS2I3 crystallize in the space group, Cmcm, with unit cell parameters a = 4.3214 (9), b = 14.258 (3), and c = 16.488 (3) Å; a = 4.2890 (6), b = 14.121(2), and c = 16.414 (3) Å, respectively. Sn2BiSI5 adopts a unique crystal structure that crystallizes in C2/m with cell parameters a = 14.175 (3), b = 4.3985 (9), c = 21.625 (4) Å, and β = 98.90(3)°. The crystal structures of Pb2BiS2I3 and Sn2BiS2I3 are strongly anisotropic and can be described as three-dimensional networks that are composed of parallel infinite ribbons of [M4S2I4] (M = Pb, Sn, Bi) running along the crystallographic c-axis. The crystal structure of Sn2BiSI5 is a homologue of the M2BiS2I3 (M = Pb, Sn) which has two successive ribbons of [M4S2I4] separated by two interstitial (Sn1–xBixI6) octahedral units. These compounds were characterized by scanning electron microscopy, differential thermal analysis, and X-ray photoelectron spectroscopy. Pb2SbS2I3, Pb2BiS2I3, “Pb2Sb1–xBixS2I3” (x ∼ 0.4), Sn2BiS2I3 and Sn2BiSI5 are highly resistive and exhibit electrical resistivities of 3.0 GΩ cm, 100 MΩ cm, 65 MΩ cm, 1.2 MΩ cm, and 34 MΩ cm, respectively, at room temperature. Pb2BiS2I3, Sn2BiS2I3, Pb2SbS2I3, “Pb2Sb1–xBixS2I3” (x ∼ 0.4), and Sn2BiSI5 are semiconductors with bandgaps of 1.60, 1.22, 1.92, 1.66, and 1.32 eV, respectively. The electronic band structures of Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5, calculated using density functional theory, show that all compounds are direct bandgap semiconductors.