The mechanisms, pathways, and driving force for the classical $B1\text{−}B2$ transitions have long been elusive. Here, we conduct ultrafast x-ray diffraction measurements and systematic atomistic simulations of single-crystal KCl under shock compression and establish a unified model for the $B1\text{−}B2$ transition in KCl. The transition proceeds via resolved-shear-stress-driven slip through two generalized transition pathways, yielding two generalized $B1\text{−}B2$ orientation relationships. Resolved-shear-stress-driven slip is potentially a common mechanism for general $B1\text{−}B2$ transitions.

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• Phase transitions
• Shock waves
• Ultrafast phenomena
• X-ray diffraction