AbstractNanosized silicon powders were prepared by gas‐phase cluster agglomeration reactions in a low‐pressure silane plasma. The formation and agglomeration of clusters leading to the growth of primary particles of powder were studied by in‐situ techniques including mass spectroscopy and laser light‐scattering experiments. These powders, generally amorphous and crystallized in a reducing atmosphere, were studied in detail by Raman spectroscopy and high‐resolution electron microscopy, which revealed a very rough surface of as‐prepared single powder particles with structures of 1 to 2 nm. Upon 1–h annealing at temperatures between 300 and 600°C, circular contrast features, 1.5 to 2.5 nm in size, are observed in the amorphous particles, which show medium‐range order. A distinct onset of crystallization is observed at 700°C with structures ranging from very small crystalline ordered regions of 2.5–3.5 nm in size to fast‐grown multipletwinned crystallites. The crystallization behavior is influenced by the clusters that form primary particles. Observed sintering behavior cannot be explained by a classical approach; hence, theoretical models need to be adapted