Ab initio molecular dynamics (AIMD) in combination with the temperature dependent effective potential (TDEP) method has been used to go beyond the quasiharmonic approximation and study the lattice dynamics in ceria, ${\mathrm{CeO}}_2$, at finite temperature. The results indicate that the previously proposed connection between the $B_{1u}$ phonon mode turning imaginary and the transition to the superionic phase in fluorite structured materials is an artifact of the failure of the quasiharmonic approximation in describing the lattice dynamics at elevated temperatures. We instead show that, in the TDEP picture, a phonon mode coupling to the $E_u$ mode prevents the $B_{1u}$ mode from becoming imaginary. We directly observe the superionic transition at high temperatures in our AIMD simulations and find that it is initiated by the formation of oxygen Frenkel pairs (FP). These FP are found to form in a collective process involving simultaneous motion of two oxygen ions.

• Received 5 January 2018
• Revised 23 February 2018

DOI:https://doi.org/10.1103/PhysRevB.97.104309