Ultrafast X-Ray Diffraction Visualization of B1−B2 Phase Transition in KCl under Shock Compression

The classical $B1(\mathrm{NaCl})\ensuremath{\leftrightarrow}B2(\mathrm{CsCl})$ transitions have been considered as a model for general structural phase transformations, and resolving corresponding phase transition mechanisms under high strain rate shock compression is critical to a fundamental understanding of phase transition dynamics. Here, we use subnanosecond synchrotron x-ray diffraction to visualize the lattice response of single-crystal KCl to planar shock compression. Complete $B1\text{\ensuremath{-}}B2$ orientation relations are revealed for KCl under shock compression along ${⟨100⟩}_{B1}$ and ${⟨110⟩}_{B1}$; the orientation relations and transition mechanisms are anisotropic and can be described with the standard and modified Watanabe-Tokonami-Morimoto model, respectively, both involving interlayer sliding and intralayer ion rearrangement. The current study also establishes a paradigm for investigating solid-solid phase transitions under dynamic extremes with ultrafast synchrotron x-ray diffraction.