Formation of a Polar Structure in the Metallic Niobium Sulfide Nb₄S₃

The synthesis of Nb₄S₃, a previously undiscovered binary sulfide, was achieved using Nb₃Br₇S as a precursor. Its structure is composed of Nb₆S triangular prisms arranged in a polar (Imm2) configuration, with sulfur atoms lying in channels along the a axis. Electrical resistivity measurements and density functional theory calculations were used to determine that Nb₄S₃ is metallic and therefore a polar metal, with metallic bands occupied by electrons with primarily niobium character. The electrons near the Fermi level are so closely associated with the niobium sublattice that the sulfur atoms have positive Born effective charges, indicating that the electrostatic interactions between sulfur atoms are unscreened. Calculations of the dependence of the electron density on the sulfur atomic positions confirm that the metallic electrons do not screen the dipole-dipole interactions between sulfur atoms, which allows polarity and metallicity to coexist in Nb₄S₃. These findings suggest that applied electric fields might be able to reverse the polarity of thin films of Nb₄S₃.