Nanocrystalline Iron Monosulfides Near Stoichiometry

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.

On the interactions of iron sulphide with alumina and silica

“…A slight change in the Fe:S ratio results in variations in structural, chemical, and other (Li, 1997;Roberts et al, 2018). These polymorphs present different diffraction patterns because the temperature of formation and the cooling/heating rate determine the final diffraction pattern (Roberts et al, 2018). The FeS phase relations display extensive solid solution and several polymorphs, as discussed above.…”

“…Fe (1-x) S is a solid solution, and the Fe content in Fe (1-x) S can vary from 50 atom % to 46.7 atom %. A slight change in the Fe:S ratio results in variations in structural, chemical, and other (Li, 1997;Roberts et al, 2018). These polymorphs present different diffraction patterns because the temperature of formation and the cooling/heating rate determine the final diffraction pattern (Roberts et al, 2018).…”

Bulk synthesis of stoichiometric/meteoritic troilite (FeS) by high‐temperature pyrite decomposition and pyrrhotite melting

“…In previous studies, the synthesis of pyrrhotite (Fe 1− x S) or troilite (FeS) was achieved using chemical (Akhtar et al., 2013; Pedoussaut & Lind, 2008; Roberts et al., 2018) or thermal treatment (Boyabat et al., 2003; Coats & Bright, 1965; de Oliveira et al., 2018; Onufrienok et al., 2020; Selivanov et al., 2008; Xu et al., 2019). For example, the synthesis of pyrrhotite by heating pyrite at high temperature (<1100 K) in CO 2 or N 2 atmosphere (Boyabat et al., 2003; de Oliveira et al., 2018) or the synthesis of troilite from pyrite at higher temperature under vacuum (1200 K; Onufrienok et al., 2020) was successfully achieved.…”

Cation diffusion patterns across the magneto-structural transition in Fe7S8

“…Metal-deficient solid phases, mainly oxides (M 1Àx O) or sulfides (M 1Àx S), exhibit physical properties that are of broad interest in Earth and astrophysical sciences to constrain physico-chemical processes during planetary evolution, but also in materials science as a concept to design functional compounds. [1][2][3][4][5][6] These defective phases have in common that they undergo structural transitions under changing temperature and/or pressure conditions due to rearrangement of the cation-deficient sites. [7][8][9][10] Among defective solids the minerals of the pyrrhotite group (Fe 1Àx S; 0 o x r 0.125) represent a classic omission solidsolution series formed by a hexagonal close-packing of sulfur with ordered Fe sites and vacancies in the octahedral interstices that results in a hexagonal NiAs-type substructure and variable commensurate and incommensurate superstructures.…”

“…9,16,17 Because of the unique structural and physico-chemical properties of the members of the pyrrhotite group and their potential for technological applications, great effort has been put into the synthesis of pyrrhotite nanostructures such as nanotubes and nanowires. 5,[18][19][20][21] The inherent difficulty associated with pyrrhotite is the synthesis of structurally and magnetically well-defined omission phases with highly ordered and stable arrangements of empty sites. Therefore, apart from a fundamental physio-chemical interest, detailed information on the cation-diffusion processes is a requirement on the route to the technological application of the pyrrhotite phases.…”