Ce And Ceo <sub> 2 </ Sub> Phase Transition And Phase Stability: Lattice Dynamics

ab initio pseudopotentials plane waves method supplemented with the linear response approach, as well as the high-pressure Raman scattering, Brillouin scattering, and infrared spectroscopy, has been employed to study the lattice dynamics of cerium and its dioxide. The bulk properties, entire phonon dispersions, and temperature-dependent thermodynamic quantities of both cerium phases and dioxide are obtained.With regard to the phonon spectrum of α-Ce, it exhibits negative phonon frequencies near the Γ point, indicating that this phase is unstable under small ambient pressure. The calculated phonon spectrum and elastic constants of γ-Ce successfully reproduce several extraordinary features, including pronounced phonon softening at L point, particularly low energy phonon along T[ξξξ] branch, and large elastic anisotropy. The phonon spectrum of δ-Ce displays negative frequencies and phonon softening over abroad region of the wave vectors. It is believed that the δ phase is totally unstable at zero temperature, and it turns to be stabilized at high temperature mainly due to the contribution of phonon entropy. Some apparent discrepancies have been observed between the phonon density of states of α and γ-Ce. As a result, the phonon entropy change across the α-γ phase boundary is not negligible. To obtain a full understanding of the driving force of the fascinating α-γ phase transition, the lattice contribution should be taken into consideration carefully.According to the spectra of high-pressure Raman scattering, we can infer that the pressure-induced phase transition of CeO₂ occurs around 30 GPa, which is likely associated with the negative phonon frequencies near the X point in the phonon dispersions of it. More satisfactory, the calculated vibrational frequencies of active Raman and infrared modes are in well accordance with the corresponding experiments.Finally, we compare the lattice dynamical properties of γ-Ce, δ-Ce, and CeO₂ to those of δ-Pu, e-Pu, and PuO₂ coarsely. It highlights that Ce (or CeO₂) can be taken as the potential surrogate for Pu (or PuO₂). We also forecast the main peaks of Raman scattering and infrared absorption spectra of PuO₂ by first principles merely, and suggest that a similar pressure-induced phase transition should be observed for PuO₂...