Understanding the dynamic response of Ce-La alloys under extreme pressures is critical for unraveling their complex high-pressure behaviors and establishing accurate multiphase equations of states (EOSs). While existing studies focus on pressures below 20 GPa, the shock response at higher pressures remains poorly characterized. Here, we conducted front-surface-impact (FSI) shock experiments on a Ce-5 wt.% La alloy dynamically compressed to 100 GPa, enabling direct measurements of the Hugoniot curve, longitudinal and bulk sound velocities $(C_L \text{and}–C_B)$, and Grüneisen parameter $\left(\gamma \right)$. Our results revealed distinct slope changes in shock velocity $\left(u_s\right)$ and $C_B$ as functions of particle velocity $\left(u_p\right)$, accompanied by abrupt discontinuities in $C_L$ and $\gamma$. These observations, coupled with elastic-plastic to fully plastic release behavior and the merging of $C_L$ into $C_B$, provide strong evidence of shock-induced melting, allowing us to determine the melting boundary initiating at $\sim 13$ GPa and completing below 21.91 GPa. Notably, in the liquid phase, $\gamma$ exhibits an anomalous increase with density $\left(\rho \right)$, which deviates from the conventional assumption $\gamma \rho \cong {\gamma }_0{\rho }_0$. Furthermore, La additives were found to enhance sound velocities and elevate both the onset and completion pressures of melting compared to pure Ce. The experimental $P\text{−}V$ and $u_s\text{−}{u}_p$ data confirm the validity of the law of additive volumes for ideal mixtures in describing the liquid-phase response up to 100 GPa. These findings advance the understanding of multiphase properties in Ce-based alloys under dynamic compression and elucidate the regulatory role of La additives. They provide critical insights for constructing wide-range multiphase EOSs and studying phase-transition kinetics in heavy actinide surrogates.

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• Equations of state
• Optical interferometry
• Shock waves
• Solid-solid transformations
• Alloys