Abstract
Solids composed of iron and sulfur are earth abundant and nontoxic, and can exhibit interesting and technologically important optical, electronic, and magnetic phenomena. However, the iron-sulfur (Fe-S) phase diagram is congested in regions of slight non-stoichiometric iron vacancies, and even when the iron atomic composition changes by even a few percent at standard temperature and pressure, there are myriad stable crystal phases that form with qualitatively different electronic properties. Here, we synthesized and characterized nanocrystals of the pyrrhotite-4M structure (Fe7S8) in an anhydrous oleylamine solvent. Upon heating from 140 °C to 180 °C, the solid sequentially transformed into two kinetically trapped FeS intermediate phases before reaching the pyrrhotite-4M final product. Finally, we assessed the effects of iron vacancies using the stoichiometric end-member, troilite, as a reference system. Density functional theory calculations show that iron vacancies in troilite shift the structure from hexagonal FeS to a monoclinic structure, similar to crystal structures of pyrrhotites, and suggest that this iron deficient troilite may be a stable intermediate between the two crystal structures. The calculations predict that defects also close the band gap in iron deficient troilite.
Highlights
Iron sulfide solids play central roles in biological processes, catalysis, planetary science, prebiotic chemistry and geochemistry[1,2,3,4,5]
We provide a comprehensive analysis of the crystal structure and morphology using x-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and high-resolution transmission electron microscopy (HRTEM)
We performed all density functional theory (DFT) calculations using the Vienna Ab initio Simulation Package (VASP)[32,33,34,35] and employed the exchange-correlation functional based on the Perdew Burke-Ernzerhof (PBE) form of the generalized gradient approximation (GGA)[36], augmented with a rotationally-invariant Hubbard-like U term to account for strong “on-site” electron correlation on the iron 3d orbitals
Summary
Iron sulfide solids play central roles in biological processes, catalysis, planetary science, prebiotic chemistry and geochemistry[1,2,3,4,5]. Iron defects in these materials, even at a concentration of a few percent, affect their properties in poorly understood ways[21,22,23] These complexities have plagued a comprehensive structural and chemical characterization of nearly stoichiometric FeS, as well as systems for which FeS is a precursor, namely iron pyrite (FeS2), a theoretically promising material for photovoltaic applications. Understanding and quantifying their nature systematically is a principal component of the work presented here, and is a necessary step in rational synthetic control over iron sulfide compounds in general. Reported compositions of iron deficient troilite have off-stoichiometries comparable to some pyrrhotites even though troilite technically denotes a stoichiometric FeS structure[26]
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