Abstract
AbstractPyrrhotites, characterized by the chemical formula Fe1–δS (0 < δ ≤ 1/8), represent an extended group of minerals that are derived from the NiAs-type FeS aristotype. They contain layered arrangements of ordered Fe vacancies, which are at the origin of the various magnetic signals registered from certain natural rocks and can act as efficient electrocatalysts in oxygen evolution reactions in ultrathin form. Despite extensive studies over the past century, the local structural details of pyrrhotite superstructures formed by different arrangements of Fe vacancies remain unclear, in particular at the atomic scale. Here, atomic-resolution high-angle annular dark-field imaging and nanobeam electron diffraction in the scanning transmission electron microscope are used to study natural pyrrhotite samples that contain commensurate 4C and incommensurate 4.91 ± 0.02C constituents. Local measurements of both the intensities and the picometer-scale shifts of individual Fe atomic columns are shown to be consistent with a model for the structure of 4C pyrrhotite, which was derived using X-ray diffraction by Tokonami et al. (1972). In 4.91 ± 0.02C pyrrhotite, 5C-like unequally sized nano-regions are found to join at anti-phase-like boundaries, leading to the incommensurability observed in the present pyrrhotite sample. This conclusion is supported by computer simulations. The local magnetic properties of each phase are inferred from the measurements. A discussion of perspectives for the quantitative counting of Fe vacancies at the atomic scale is presented.
Highlights
Pyrrhotite (Fe1–δS; 0 < δ ≤ 1/8) is one of the most common metal sulfide minerals in the Earth’s ore deposits, as well as in a range of meteorites (Rochette et al 2001, 2005; Weiss et al 2002; Lorand et al 2005; Yu and Gee 2005; Louzada et al 2007)
Pierce and Buseck (1974) studied incommensurate pyrrhotites using HRTEM dark-field imaging and described the superstructures in terms of disordered sequences of anti-phase domains. This description was extended by Harries et al (2011), who proposed a translation interface modulation (TIM) model on the basis of electron diffraction and HRTEM dark-field imaging results
Lamellar transmission electron microscopy (TEM) specimens were prepared from its polished top surface using focused ion beam (FIB) milling with Ga in an FEI Helios Nanolab 400s dual beam system (ER-C 2016a)
Summary
Pyrrhotite (Fe1–δS; 0 < δ ≤ 1/8) is one of the most common metal sulfide minerals in the Earth’s ore deposits, as well as in a range of meteorites (Rochette et al 2001, 2005; Weiss et al 2002; Lorand et al 2005; Yu and Gee 2005; Louzada et al 2007). The slight change in composition from stoichiometric FeS due to the incorporation of vacancies and the resulting vacancy ordering lead to the formation of different superstructures, which are referred to as NC pyrrhotites and are typically characterized by unit cells that are multiples of the parent NiAs-type subcell In this notation, N is the coefficient of the c lattice parameter [see Pósfai et al (2000) for a description of the structural relationships]. Pierce and Buseck (1974) studied incommensurate pyrrhotites using HRTEM dark-field imaging and described the superstructures in terms of disordered sequences of anti-phase domains This description was extended by Harries et al (2011), who proposed a translation interface modulation (TIM) model on the basis of electron diffraction and HRTEM dark-field imaging results. The present study of the 4.91 ± 0.02C superstructure provides a high-quality model that can be used to assess the use of the 4D superspace formalism for pyrrhotite, as well for other omission/defect structures
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