The polar mixed-valent lanthanum iron(II, III) sulfide La 3Fe 2−δS 7: Synthesis, crystal and electronic structure, 57Fe Mößbauer spectra, magnetic susceptibility and electrical resistivity

Abstract

La 3Fe 2− δ S 7 ( 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 P6 3 with a=10.1906(6), c=5.9543(4) Å and Z=2, and adopts the Ce 6Al 10/3S 14 structure type. The structure contains both octahedral and tetrahedral iron sites: one-dimensional rods of face-sharing FeS 6 octahedra run along the 6 3 screw axis of the cell; FeS 4 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 T N≈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 FeS 6 octahedra. However, due to the defects on the Fe II positions, La 3Fe 2− δ S 7 shows an activated conducting behavior.