Ba4FeAgS6: a new antiferromagnetic and semiconducting quaternary sulfide

The single crystals of a quaternary sulfide, Ba₄FeAgS₆, have been synthesized by reacting elements at 873 K inside a sealed fused silica tube. The title phase is the first ordered quaternary compound of the Ba-Ag-Fe-S system. The crystal structure of Ba₄FeAgS₆ is characterized by a single-crystal X-ray diffraction study at 298(2) K. It crystallizes in the space group C52h - P2₁/n of the monoclinic crystal system with unit cell dimensions of a = 8.6367(5) Å, b = 12.0291(7) Å, c = 13.2510(7) Å, and β = 109.015(2)°. This compound is stoichiometric, and its structure contains twelve unique crystallographic sites: four Ba, one Fe, one Ag, and six S sites. All atoms of the structure occupy the general positions. The Ba₄FeAgS₆ structure consists of one-dimensional chains of 1∞[FeAgS₆]^8- that are extended in the [100] direction. The negative charges on these chains are counterbalanced by the filling of Ba²⁺ cations in between the 1∞[FeAgS₆]^8- chains. The Fe atoms are bonded to four S atoms that form a distorted tetrahedral geometry around the central Fe atom. Each Ag atom in this structure is coordinated with four S atoms in a distorted tetrahedral fashion. These FeS₄ and AgS₄ motifs are the main building blocks of the Ba₄FeAgS₆ structure. The corner-sharing of FeS₄ and AgS₄ tetrahedra creates one-dimensional chains of 1∞[FeAgS₆]^8-. This structure does not contain any homoatomic or metallic bonds and can be charge-balanced as (Ba²⁺)₄(Fe³⁺)₁(Ag¹⁺)₁(S^2-)₆. The optical absorption study performed on a polycrystalline Ba₄FeAgS₆ sample reveals a direct bandgap of 1.2(1) eV. The magnetic studies reveal the antiferromagnetic behavior of Ba₄FeAgS₆ below 50 K. The thermal conductivity and theoretical electronic structure of Ba₄FeAgS₆ are also studied in detail.

NaOH-Intercalated Iron Chalcogenides (Na1-xOH)Fe1-yX (X = Se, S): Ion-Exchange Synthesis and Physical Properties

The discovery of the (Li_1-xFe_xOH)FeSe superconductor has aroused significant interest in metal hydroxide-intercalated iron chalcogenides. However, all efforts made to intercalate NaOH between FeSe and FeS layers have failed so far. Here we report two NaOH-intercalated iron chalcogenides (Na_1-xOH)Fe_1-yX (X = Se, S) that were synthesized by a low-temperature hydrothermal ion-exchange method. Their crystal structures were solved through single-crystal X-ray diffraction and refined against powder X-ray and neutron diffraction data. Different from the (Li_1-xFe_xOH)FeX superconductors that crystallize in a tetragonal space group P4/nmm with Z = 2, (Na_1-xOH)Fe_1-yX belong to an orthorhombic space group Cmma with Z = 4. The structural solution also reveals that there are vacancies in both Na and Fe sites and there are not iron ions in the (Na_1-xOH) layer. This is probably why both Fe(II) and Fe(III) species exist in the title compounds, as detected by X-ray photoelectron spectroscopy. Based on magnetization and electrical resistivity measurements, the two compounds were found to be paramagnetic semiconductors. The absence of superconductivity should be closely related to the iron vacancies in the Fe_1-yX layer. Theoretical calculations suggest that inducing superconductivity in (Na_1-xOH)Fe_1-ySe is promising due to the similarity of the electronic structures between stoichiometric (NaOH)FeSe and the (Li_1-xFe_xOH)FeSe superconductor.

Synthesis of compositionally tunable, hollow mixed metal sulphide CoxNiySz octahedral nanocages and their composition-dependent electrocatalytic activities for oxygen evolution reaction

Hollow nanostructures such as nanocages and nanoframes can serve as advanced catalysts with their enlarged active surface areas, and hence they have been of widespread interest. Despite the recent progress in the synthesis of this class of nanomaterials, hollow nanostructures with tunable compositions and controlled morphologies have rarely been reported. Here, we report a facile synthetic route to a series of compositionally tunable, hollow mixed metal sulphide (Co_xNi_yS_z) octahedral nanocages. The sulfidation of CoO octahedral nanoparticles generates CoO@Co_xS_y core-shell octahedra, and the in situ etching of the CoO core and annealing yield Co₉S₈ (pentlandite) octahedral nanocages (ONC). The addition of a Ni precursor during the etching/annealing process of CoO@Co_xS_y core-shell octahedra progressively yields hollow ONC structures of Co_9-xNi_xS₈, Ni₉S₈, Ni₉S₈/β-NiS, and Ni₃S₂/β-NiS via cation exchange reactions. Mixed cobalt/nickel sulphide, Co_9-xNi_xS₈ ONC, shows superior oxygen evolution reaction activity to monometallic sulphide ONC structures, demonstrating the synergy between different metal species.