Anharmonicity in lattice dynamics of Ce at the γ-α phase transition

The nature of the γ and α phases in Ce, and the intriguing isostructural γ -α phase transition, are challenging and highly debated topics in condensed matter physics, which are usually attributed to the strong correlation of localized 4f electrons. On the other hand, lattice dynamics might also play an important role in this issue. However, there have been few studies devoted to exploring the effects of lattice dynamics in Ce, particularly the possible role of anharmonicity. In this work, a thorough and systematic investigation of anharmonicity in the lattice dynamics of Ce near the γ-α isostructural transition is carried out using the ab initio self-consistent phonon method. The required potential energy surface (PES) is obtained using DFT with the exchange-correlation (XC) functional selected by assessing various functionals. By taking the experimental isothermal equation of state (EOS) as the benchmark, we validate that the semi-local PBEsol functional is accurate enough to model the γ phase when the fully localized 4f electrons are placed in the core; on the other hand, the PBE functional with the 4f electrons in the valence space provides good agreement with the experimental data for the α phase. This practical approximation has the advantage of avoiding the explicit treatment of the strong correlations of localized 4f states, and leads to much higher computational efficiency if only forces are considered. The lattice dynamics and anharmonicity in the γ and α phases of cerium are investigated using the temperature-dependent effective potential (TDEP) and full temperature-dependent potential (FTDP) methods. Both methods reveal significant contributions from anharmonicity, especially the quantum sampling in FTDP, which brings the theoretical data into good agreement with the experimental data. Our results demonstrate that anharmonicity plays an important role in the lattice dynamics of cerium. These insights might be crucial for understanding the compression behavior of Ce and for developing efficient and accurate interatomic potentials for similar systems for large-scale molecular dynamics simulations.