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 Fe_1-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 ~400–450^oC, which is ~100–150^oC 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.

黄铁矿（FeS ₂）具有动态热稳定性，是探测材料中复杂相变的理想模型系统。在升高的温度下，S原子从FeS ₂晶格升华导致相变为黄铁矿的多种多晶型形式之一，即。，选自Fe _1-x S（其中0≤X <0.2），以化学计量的FeS。这种相变的复杂性质在纳米尺度上仍然相对未被探索。在这里，我们使用原位透射电子显微镜（TEM）和原位X射线衍射（XRD）在真空下观察〜150 nm黄铁矿纳米粒子到黄铁矿的相变。虽然纳米颗粒的整体形状仍然是立方体，纳米颗粒的晶体结构由立方黄铁矿变为六角磁黄铁矿在〜400-450 ^Ô C，其为〜100-150 ^ö下比散装黄铁矿的报告值降低。有趣的是，我们的原位观察结果没有证据表明已经有核-壳转变模型用于大块黄铁矿向黄铁矿的相变，表明较短的长度尺度在纳米粒子中扩散具有重要作用，而S空位的作用则有助于更快地原子扩散和重排。此外，发现升温速率会影响相变温度，而较低的转变温度会影响较高的升温速率，这表明动力学效应对相变的作用。我们希望我们的详细见解将有助于推动FeS ₂ 纳米颗粒在高温应用（包括催化和高温电池）中的使用。