Soft anharmonic coupled vibrations of Li and SiO4 enable Li-ion diffusion in amorphous Li2Si2O5

Abstract

We present investigations on atomic dynamics and Li⁺ diffusion in crystalline and amorphous Li₂Si₂O₅ using quasielastic neutron scattering (QENS) and inelastic neutron scattering (INS) studies supplemented by ab initio molecular dynamics simulations (AIMD). The QENS measurements in the amorphous phase of Li₂Si₂O₅ show a narrow temperature window (700 < T < 775 K), exhibiting significant quasielastic broadening corresponding to fast Li⁺ diffusion. Our INS measurements clearly show the presence of large phonon density of states (PDOS) at low energy (low-E) in the superionic amorphous phase, which disappear in the non-superionic crystalline phase, corroborating the role of low-E modes in Li⁺ diffusion. The frustrated energy landscape and host flexibility (due to random orientation and vibrational motion of SiO₄ polyhedral units) play an essential role in diffusing Li⁺. We used AIMD simulations to identify that these low-E modes involve a large amplitude of Li vibrations coupled with SiO₄ vibrations in the amorphous phase. At elevated temperatures, these vibrational dynamics accelerate Li⁺ diffusion. Above 775 K, these SiO₄ vibrational dynamics drive the system into the crystalline phase by locking SiO₄ and Li⁺ into deeper minima of the free energy landscape and making them disappear in the crystalline phase. Both experiments and simulations provide valuable information about the atomic level stochastic and vibrational dynamics in Li₂Si₂O₅ and their role in Li⁺ diffusion and vitrification.