  Silicon (Si) is one of the most abundant elements on Earth, and it is the
most important and widely used semiconductor, constituting the basis of modern
electronic devices. Despite extensive study, some properties of Si remain
elusive. For example, the behaviour of Si under high pressure, in particular at
the ultra-high strain rates characteristic of dynamic compression, has been a
matter of debate for decades. A detailed understanding of how Si deforms is
crucial for a variety of fields, ranging from planetary science to materials
design. Simulations suggest that in Si the shear stress generated during shock
compression is released inelastically, i.e., via a high-pressure phase
transition, challenging the classical picture of relaxation via defect-mediated
plasticity. However, experiments at the short timescales characteristic of
shock compression are challenging, and direct evidence supporting either
deformation mechanism remain elusive. Here, we use sub-picosecond,
highly-monochromatic x-ray diffraction to study (100)-oriented single-crystal
Si under laser-driven shock compression. We provide the first unambiguous,
time-resolved picture of Si deformation at ultra-high strain rates,
demonstrating the predicted inelastic shear release. Our results resolve the
longstanding controversy on silicon deformation under dynamic compression, and
provide direct proof of strain rate-dependent deformation mechanisms in a
non-metallic system, which is key for the study of planetary-relevant
materials.
