Ultrafast X-Ray Diffraction Visualization of $B1\text{−}B2$ Phase Transition in KCl under Shock Compression

The classical $B1(\mathrm{NaCl})\leftrightarrow B2(\mathrm{CsCl})$ 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 $B1\text{−}B2$ 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.

Figure 1

Schematic of experimental setup. (a) Gas-gun shock compression loading with sub-ns x-ray diffraction measurements (see Sec. S2 in the Supplemental Material [28] for details). The probe x-ray pulse width is approximately 80 ps, and the pulse interval is about 153 ns. DPS: Doppler pin system or Doppler laser interferometry. (b) ${[110]}_{B1}$- and ${[100]}_{B1}$-oriented single-crystal KCl targets. The impact loading axis (LA) is perpendicular to the $(110{)}_{B1}$ or $(100{)}_{B1}$ lattice plane (red). (c) Representative free-surface velocity history of the $[110{]}_{B1}$ target measured with DPS, showing a plastic wave ($P$), a phase transition wave (PT), and a phase interface reflection wave (PIR) [45]. Here, time zero refers to the shock breakout at the free surface.

Figure 2

Shock compression of single-crystal KCl along $[110{]}_{B1}$. (a) Measured static XRD pattern of the $B1$ phase before impact. (b) Simulated static XRD pattern of the $B1$ phase corresponding to (a). (c) Measured dynamic XRD pattern (323 ns after impact), showing diffraction spots from both the $B1$ and $B2$ phases. (d) Simulated dynamic XRD pattern of the $B2$ phase corresponding to (c). Diffraction spots from the $B1$ phase are not shown in (d) for clarity. White Miller indices: the $B1$ phase; orange Miller indices: the $B2$ phase. Subscript 2: diffraction spots due to the second harmonic of the synchrotron undulator source. White circles: low-intensity spots. PC: scattered x rays from the polycarbonate projectile and windows.

Figure 3

$B1\to B2$ phase transition mechanisms in single-crystal KCl under shock compression along $[110{]}_{B1}$. (a) Initial crystal structure of the $B1$ phase. The virtual $B2$ unit cell within the $B1$ lattice is indicated in black lines. (b) Resultant structure of the $B2$ phase. The $B2$ unit cell is indicated in black lines. (c) and (d) Crystal structures of the $B2$ phase for mechanisms I and II, respectively. Blue: $K^{+}$; orange: ${\mathrm{Cl}}^{-}$. Arrows: sliding directions.