The polar mixed-valent lanthanum iron(II, III) sulfide La₃Fe_2−δS₇: Synthesis, crystal and electronic structure, ⁵⁷Fe Mößbauer spectra, magnetic susceptibility and electrical resistivity

Abstract

La₃Fe_2−δS₇ (δ=0.042(6)) was synthesized through a reaction of the elements in a LiCl/KCl flux at 970 K, and its structure was determined by single-crystal X-ray diffraction. The compound crystallizes in the polar hexagonal space group P6₃ with a=10.1906(6), c=5.9543(4) Å and Z=2, and adopts the Ce₆Al_10/3S₁₄ structure type. The structure contains both octahedral and tetrahedral iron sites: one-dimensional rods of face-sharing FeS₆ octahedra run along the 6₃ screw axis of the cell; FeS₄ tetrahedra, all pointing in the same direction, are stacked along the threefold rotation axes. The iron-centered polyhedra are linked by lanthanum atoms, which are coordinated by [7+1] sulfur atoms in a bicapped trigonal prismatic arrangement. ⁵⁷Fe Mößbauer spectroscopy confirms that Fe^III and Fe^II cations occupy the tetrahedral and octahedral iron sites, respectively. Magnetic susceptibility data indicate an antiferromagnetic transition at TN≈155 K. Density functional band structure calculations within the local density approximation reveal two covalent Fe–S subsystems within the compound that mix only weakly. A large anisotropy is indicated by bands that disperse predominantly along the hexagonal axis. The electronic band structure suggests pseudo-one-dimensional metallic conductivity along the rods of face-sharing FeS6 octahedra. However, due to the defects on the Fe^II positions, La₃Fe_2−δS₇ shows an activated conducting behavior.