Compositional observations of franckeite using high-angle annular dark-field microscopy

Franckeite (˜FeSn3Pb5Sb2Si14) has a complex modulated structure containing alternating layers of different composition. The complex composition and structure, lack of a well-defined unit cell, incommensurate character, and poor quality of crystals has made it difficult to study by standard X-ray and electron-diffraction methods. High-angle annular dark-field (HAADF) STEM imaging provides a way of mapping the compositional distributions directly and with atomic resolution. For thin regions of the sample, the intensity of the electrons scattered at high angles depends strongly on the atomic number of the scatterer. This unique property makes HAADF imaging powerful for studying compositional modulations on the atomic scale. By comparing the structure as viewed by high-resolution imaging to the compositional information provided by the HAADF technique, we have been able to obtain an improved understanding of crystal structures with displacive and compositional modulations, such as franckeite. We use the HAADF technique to test the assumption that in franckeite two kinds of layers (SnS2 and PbS) are stacked along the a axis with a 1.73 nm repeat, and that Sb and Fe produce structural modulations in the c direction.