Syntheses, crystal structures and characterizations of two new quaternary thioborates: PbMBS4 (M = Sb, Bi)

Abstract

Two new quaternary thioborates, PbSbBS₄ and PbBiBS₄, have been synthesized from solid-state reaction methods at temperatures from 1073 to 1123 K in evacuated sealed quartz tubes. The crystal structures have been determined by means of single crystal X-ray diffraction and they both crystallize in the P2₁/m space group of the monoclinic system with a = 5.9532(18) Å, b = 6.2031(13) Å, c = 9.250(3) Å, β = 108.200(16)°, Z = 2 for PbSbBS₄ and a = 5.971(10) Å, b = 6.273(9) Å, c = 9.132(15) Å, β = 107.75(2)°, Z = 2 for PbBiBS₄, respectively. The two compounds are isostructural and both constructed with the infinite one-dimensional [MBS₄]²⁻ (M = Sb or Bi) chains as building blocks, which are composed of [BS₃]³⁻ trigonal plane units with [MS₃]³⁻ (M = Sb or Bi) trigonal pyramids connected alternatively through corner-sharing along the crystallographic b axis. Two adjacent [MBS₄]²⁻ chains are further bridged by the intermediate Pb²⁺ cations, forming a novel S-shaped Pb-[MBS₄] dimeric chain structure. In addition, first-principles electronic structure calculations based on the density functional theory (DFT) were performed on compound PbSbBS₄, indicating that the compound belongs to direct semiconductor with a band gap of 1.803 eV, which is in good agreement with the experimental value estimated from the UV-Vis diffuse reflectance spectroscopy.