Observation of specific optical phonon modes dominating Li ion diffusion in γ-LiAlO2 ceramic

Abstract

In γ-LiAlO2 ceramic, Li ion diffusion plays a key role both in tritium release as a solid tritium breeder material in nuclear fusion and in phase stability as a matrix material in molten carbonate fuel cells (MCFC). Yet fundamental understanding of the diffusive process in γ-LiAlO2 is still missing, especially considering the interaction of the lattice dynamics and Li ion motion. Herein, we demonstrated that two specific optical phonon modes are coupled with Li ion diffusion in γ-LiAlO2, by investigating the temperature-dependent lattice dynamics via Raman scattering experiments, as well as first-principles calculations and neutron diffraction experiments (ND). The high-temperature ND experiments showed that γ-LiAlO2 undergoes partial Li ion disordering in Li sublattice at high temperature. Notably, the lattice dynamics studies demonstrated that B1 mode at 226 cm−1 and A1 mode at 263 cm−1 are responsible for Li ion motion through the oscillation of Li–O in <010> and <001> direction, respectively. Our results further suggest that the behavior of tritium release and phase stability in γ-LiAlO2 at elevated temperature may be controlled by tuning the two specific phonon modes through photon/electron-phonon coupling.