Rare-earth compounds $R\text{CoSi}$ exhibit distinct structural behaviors depending on whether $R$ is a light, middle, or heavy rare-earth element. Notably, CeCoSi exhibits a structural phase transition under pressure, where the transition pressure increases with temperature. Experimental evidence indicates a direct correlation between this transition and the delocalization of $\text{Ce-4}f$ electrons. In this work, we systematically study the evolution of the electronic structure of CeCoSi with temperature and pressure using density functional theory (DFT) + dynamical mean field theory (DMFT). Our results reveal the Kondo metal behavior of CeCoSi at ambient pressure, with hybridization between $\mathrm{Ce}\text{−}4f$ and $\mathrm{Co}\text{−}3d$. Under high pressure, CeCoSi exhibits extremely strong hybridization of $\mathrm{Ce}\text{−}4f$ and $\mathrm{Co}\text{−}3d$ even at high temperatures, while the effective mass and occupation number of $\mathrm{Ce}\text{−}4f$ are significantly reduced. This indicates that pressure causes CeCoSi to transform from a Kondo metal to a mixed-valence state. The quantum transition pressure $P_{\mathrm{Q}}$, though slightly higher than the experimental structural transition pressure $P_{\mathrm{S}}$, shows identical temperature dependence. Notably, the mixed-valence state exhibits significantly enhanced hybridization strength compared to the Kondo metal state. Our results suggest that the valence instability of $\mathrm{Ce}\text{−}4f$ by pressure is the cause of the structural phase transition, possibly by inducing electron density redistribution that destabilizes the lattice.

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• Quantum phase transitions
• Rare-earth alloys
• Strongly correlated systems
• DFT+DMFT