Sub-picosecond x-ray diffraction was used to measure (100)-oriented silicon under laser-driven shock compression, providing an unambiguous atomistic picture of silicon phase transitions. We determine the orientation relationship between the Si-V and Si-I phases, and connect it with the specific deformation mechanism. We provide the first direct evidence of the inelastic deformation of Si under laser-driven shock compression, i.e., the shear stress is relieved by the phase transition without the occurrence of defect-mediated plasticity. We also demonstrate metastability of the high-pressure Si-II phase down to ambient pressure, which could lead to the synthesis of novel functional materials.
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