Synthetic iron pyrite across length scales: interfacial defects and macroscopic properties

Iron pyrite (FeS2) has long represented a materials science challenge because of the immense sensitivity of its macroscopic behavior to defect structure. Research along this vein has primarily focused on the deleterious effects of sulfur vacancies on optical and electronic properties that prevent the material from being successfully integrated into photovoltaic devices. However, despite the well-established body of literature on semiconductor behavioral changes in the presence of grain boundaries, relatively little information exists addressing the existence of internal interfaces in FeS2 and their effect on macroscopic material properties. Here we use high-resolution synchrotron X-ray diffraction to characterize the structure and composition of synthetic FeS2 particles. Particles range in size and degree of polycrystallinity to investigate the surface and interfacial effects frequently associated with an imperfect material. We assess the magnetic and optical responses using SQUID magnetometry and ultraviolet-visible light absorption spectroscopy to better understand the macroscopic effects of grain boundaries on system properties. All particle sizes, ranging from nanometer to micrometer scale, are stoichiometric and phase-pure, with grain boundary density increasing with size. Additionally, there is a correlation between increasing particle size, magnetism onset, and decreasing optical band gap, which are explained by the increased interface presence identified in our structural analysis. Magneto-optical responses provide insight on the potential defect structures present at interfacial boundaries within FeS2 solids.