Shock induced phase change in KCl single crystals: Orientation relations between the $B1$ and $B2$ lattices

The relative orientations between the lattices of the low pressure $(B1)$ and high pressure $(B2)$ phases of shock compressed KCl single crystals were examined using plate impact loading along the [111] and the [110] directions. The $B2$-phase lattice planes, perpendicular to the loading direction, were determined from transient x-ray diffraction measurements. Two closely spaced diffraction peaks were observed for the [111] loading direction. The lower Bragg angle peak is consistent with a 200 peak of a cubic $B2$-phase unit cell giving the orientation relation $[111]B1$ is parallel to $[100]B2$⁠. The higher Bragg angle peak is not consistent with any peak from either a cubic $B1$ or a cubic $B2$ unit cell; the origin of this peak is unknown. Other experiments found no orientation relations; for the [111] loading direction, $[111]B1$ is not parallel to $[211]B2$ and for the [110] loading direction, $[110]B1$ is not parallel to $[100]B2$⁠. The orientation relation determined for the [111] loading in this work is incompatible with a previously determined orientation relation obtained for the [100] loading [T. d’Almeida and Y. M. Gupta, Phys. Rev. Lett. 85, 330 (2000)]. This finding suggests that the transformation pathway between the $B1$ and $B2$ lattices in KCl crystals depends on the shock compression direction for the $B1$ phase. The results are discussed in terms of the compatibility between the macroscopic uniaxial strain imposed by shock wave loading and the microscopic rearrangement of atoms leading to the observed orientation relations.