Direct Gap Semiconductors Pb₂BiS₂I₃, Sn₂BiS₂I₃, and Sn₂BiSI₅

Abstract

New quaternary thioiodides Pb₂BiS₂I₃, Sn₂BiS₂I₃, and Sn₂BiSI₅ have been synthesized\nby isothermal heating as well as chemical vapor transport. Pb₂BiS₂I₃ and Sn₂BiS₂I₃ crystallize in the space group, <i>Cmcm</i>, with unit cell parameters <i>a</i> = 4.3214 (9), <i>b</i> = 14.258 (3), and <i>c</i> = 16.488 (3) Å; <i>a</i> = 4.2890 (6), <i>b</i> = 14.121(2), and <i>c</i> = 16.414 (3) Å, respectively. Sn₂BiSI₅ adopts a unique crystal structure that crystallizes in <i>C</i>2<i>/m</i> with cell parameters <i>a</i> = 14.175 (3), <i>b</i> = 4.3985 (9), <i>c</i> = 21.625 (4) Å, and β = 98.90(3)°. The crystal structures\nof Pb₂BiS₂I₃ and Sn₂BiS₂I₃ are strongly anisotropic and can be described\nas three-dimensional networks that are composed of parallel infinite\nribbons of [M₄S₂I₄] (M = Pb, Sn,\nBi) running along the crystallographic <i>c</i>-axis. The\ncrystal structure of Sn₂BiSI₅ is a homologue\nof the M₂BiS₂I₃ (M = Pb, Sn) which\nhas two successive ribbons of [M₄S₂I₄] separated by two interstitial (Sn_1–<i>x</i>Bi_<i>x</i>I₆) octahedral units.\nThese compounds were characterized by scanning electron microscopy,\ndifferential thermal analysis, and X-ray photoelectron spectroscopy.\nPb₂SbS₂I₃, Pb₂BiS₂I₃, “Pb₂Sb_1–<i>x</i>Bi_<i>x</i>S₂I₃” (<i>x</i> ∼ 0.4), Sn₂BiS₂I₃ and Sn₂BiSI₅ are\nhighly resistive and exhibit electrical resistivities of 3.0 GΩ\ncm, 100 MΩ cm, 65 MΩ cm, 1.2 MΩ cm, and 34 MΩ\ncm, respectively, at room temperature. Pb₂BiS₂I₃, Sn₂BiS₂I₃, Pb₂SbS₂I₃, “Pb₂Sb_1–<i>x</i>Bi_<i>x</i>S₂I₃” (<i>x</i> ∼ 0.4), and\nSn₂BiSI₅ are semiconductors with bandgaps of\n1.60, 1.22, 1.92, 1.66, and 1.32 eV, respectively. The electronic\nband structures of Pb₂BiS₂I₃, Sn₂BiS₂I₃, and Sn₂BiSI₅, calculated using density functional theory, show that all compounds\nare direct bandgap semiconductors.