Symmetry breaking of paracrystalline topology in amorphous silicon

The atomic structure of amorphous Si (a-Si) has traditionally been described by the continuous random network (CRN) model, which consists of the four-coordinated Si with a non-periodic structure. However, the paracrystalline model, consisting of strained nanocrystals embedded within a disordered matrix, has gained traction. This shift is largely driven by fluctuation electron microscopy observations, which reveal the distinct diffraction patterns that are inconsistent with the CRN model. However, the degree and nature of paracrystallinity remain unclear due to a lack of experimental approaches capable of revealing finite size effects. In this paper, we present the atomic structure of a-Si that appeared in a liquid quenched Ag-Si alloy. Fast Fourier transform and electron diffraction patterns exhibit excellent agreement with molecular dynamics simulations. Furthermore, nano-beam electron diffraction reveals distinct diffraction spots that support the paracrystalline model. Importantly, these diffraction spots violate the conventional crystallographic extinction rule, implying symmetry breaking within the paracrystalline structure. This is significant because the appearance of forbidden reflections offers direct evidence of local structural changes and provides new insight into the underlying disorder in a-Si.