In situ observations of thermally induced phase transformations in iron sulfide nanoparticles

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Iron pyrite (FeS2) exhibits dynamic thermal stability and represents an ideal model system to probe complex phase transformations in materials. At elevated temperatures, the sublimation of S atoms from the FeS2 lattice results in a phase transformation to one of the many polymorphic forms of pyrrhotite, i.e., from Fe1-XS (where 0 ≤ X < 0.2) to stoichiometric FeS. The complex nature of this phase transformation remains relatively unexplored at the nanoscale. Here, we use in situ transmission electron microscopy (TEM) and in situ X-ray diffraction (XRD) to observe the phase transformation of ~150 nm pyrite nanoparticles to pyrrhotite in vacuum. Although the overall shape of the nanoparticles remains cubic, the crystal structure of the nanoparticles changes from cubic pyrite to hexagonal pyrrhotite at ~400–450oC, which is ~100–150oC lower than reported values of bulk pyrite. Interestingly, our in situ observations do not evidence a core-shell transformation model which has been reported for the phase transformation of bulk pyrite to pyrrhotite, indicating the role of shorter length scales for diffusion in nanoparticles, as well as the role of S vacancies that facilitate faster atomic diffusion and rearrangements. In addition, the heating ramp rate was found to influence the phase transition temperature with lower temperatures of transformation seen for higher heating rates indicating the role of kinetic effects on phase transformation. We expect our detailed insights will help advance the use of FeS2 nanoparticles in high-temperature applications including catalysis and high-temperature batteries.