High pressure structural transformation of copper (Cu) is a rather complex physical process. One of the intriguing questions that are rarely discussed is the comparison between quasi-isentropic response and adiabatic response for copper lattice transition. The ambient face-centered-cubic structure of Cu is predicted to persist over 100 TPa from ab inito calculations and experimentally demonstrated to persist until 1.15 TPa in ramp compression and 150 GPa in static compression. However, a novel body-centered-cubic (BCC) order is observed merely at 180 GPa once shock compression is applied. The mechanism of body-centered-cubic phase transition occurred at low pressure under shock compression remains elusive so far and much attention is required on the dynamics in such a phase transition. In this work, we utilize the molecular dynamics method to simulate the shock compression on a copper lattice to uncover the structural transition in the atomic scale. We report the FCC–BCC phase transition occurred at 156 GPa, and lots of disordered structures are discovered in the BCC phase after impact, revealed by a series of structure analysis tools and free energy calculations. The plethora of transient disordered structures reduces the global Gibbs free energies, thus leading to the downgrade of the transition pressure in contrast to the ramp and static compression, which provides a new perspective for structural transformation under extreme conditions.

For palladium (Pd) as a typical high-pressure standard material, studying its structural changes and thermodynamic properties under extreme conditions is widely demanded and challenging. Particularly, the solid-solid phase transition process of Pd under shock loading is understood still scarcely. In this paper, using the classical molecular dynamics simulations with embedded atom method (EAM) based on the interatomic potential, we investigate the phase transition of single crystal Pd from atomic scale under shock loading. A series of structural features is observed in a pressure range of 0–375 GPa, revealing that the structure feature transforms from the initial face-centered cubic (FCC) structure to the stacking faults body-centered cubic (BCC) structure with hexagonal close-packed (HCP) structure, and finally complete melting. Under shock loading of <inline-formula><tex-math id="Z-20220123201122">\begin{document}$ \left\langle {100} \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211511_Z-20220123201122.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211511_Z-20220123201122.png"/></alternatives></inline-formula> oriented bulk Pd, we find the transformation to BCC structure can take place almost at 70.0 GPa, which is much lower than the previous static calculation result. In addition, we find that the phase transition depends on the direction initially impacting crystal. Under impacting along the <inline-formula><tex-math id="Z-20220123201132">\begin{document}$ \left\langle {110} \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211511_Z-20220123201132.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211511_Z-20220123201132.png"/></alternatives></inline-formula> direction and the <inline-formula><tex-math id="Z-20220123201127">\begin{document}$ \left\langle {111} \right\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211511_Z-20220123201127.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211511_Z-20220123201127.png"/></alternatives></inline-formula> direction, the FCC-BCC phase transition pressures increase to 135.8 GPa and 165.4 GPa, respectively. Also, the introduction of defects will increase the phase transition pressure of FCC-BCC by 20–30 GPa in comparison with perfect crystals, which is verified by the distribution of the potential energy. An interesting phenomenon that FCC-BCC transition pressure of Pd decreases under shock loading is found in this work, which provides a new theoretical insight into the application of high pressure experiments in the future.