Direct Gap Semiconductors Pb₂BiS₂I₃, Sn₂BiS₂I₃, and Sn₂BiSI₅

New quaternary thioiodides Pb 2 BiS 2 I 3, Sn 2 BiS 2 I 3, and Sn 2 BiSI 5 have been synthesized by isothermal heating as well as chemical vapor transport. Pb 2 BiS 2 I 3 and Sn 2 BiS 2 I 3 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. Sn 2 BiSI 5 adopts a unique crystal structure that crystallizes in C 2 /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 Pb 2 BiS 2 I 3 and Sn 2 BiS 2 I 3 are strongly anisotropic and can be described as three-dimensional networks that are composed of parallel infinite ribbons of [M 4 S 2 I 4 ] (M = Pb, Sn, Bi) running along the crystallographic c -axis. The crystal structure of Sn 2 BiSI 5 is a homologue of the M 2 BiS 2 I 3 (M = Pb, Sn) which has two successive ribbons of [M 4 S 2 I 4 ] separated by two interstitial (Sn 1– x Bi x I 6 ) octahedral units. These compounds were characterized by scanning electron microscopy, differential thermal analysis, and X-ray photoelectron spectroscopy. Pb 2 SbS 2 I 3, Pb 2 BiS 2 I 3, “Pb 2 Sb 1– x Bi x S 2 I 3 ” ( x ∼ 0.4), Sn 2 BiS 2 I 3 and Sn 2 BiSI 5 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. Pb 2 BiS 2 I 3, Sn 2 BiS 2 I 3, Pb 2 SbS 2 I 3, “Pb 2 Sb 1– x Bi x S 2 I 3 ” ( x ∼ 0.4), and Sn 2 BiSI 5 are semiconductors with bandgaps of 1.60, 1.22, 1.92, 1.66, and 1.32 eV, respectively. The electronic band structures of Pb 2 BiS 2 I 3, Sn 2 BiS 2 I 3, and Sn 2 BiSI 5, calculated using density functional theory, show that all compounds are direct bandgap semiconductors.