Sodium Diffusion and Dynamics in Na2Ti3O7: Neutron Scattering and Ab-initio Simulations

TL;DR

This study combines neutron scattering experiments and ab-initio simulations to investigate sodium diffusion mechanisms and dynamics in Na2Ti3O7 across a wide temperature range, revealing localized jumps and long-range diffusion pathways.

Contribution

It provides new insights into Na diffusion pathways, activation energies, and vibrational contributions in Na2Ti3O7 using combined experimental and computational approaches.

Findings

Na atoms undergo localized jumps up to 1173 K

Long-range diffusion occurs along the a-axis above 1500 K

Low-energy phonons contribute to Na vibrational amplitude

Abstract

We have performed quasielastic and inelastic neutron scattering (QENS and INS) measurements from 300 K to 1173 K to investigate the Na-diffusion and underlying host dynamics in Na2Ti3O7. The QENS data show that the Na atoms undergo localized jumps up to 1173 K. The ab-initio molecular dynamics (AIMD) simulations supplement the measurements and show 1-d long-ranged diffusion along the a-axis above 1500 K. The simulations indicate that the occupancy of the interstitial site is critical for long-range diffusion. The nudged-elastic-band (NEB) calculation confirmed that the activation energy barrier is lowest for diffusion along the a-axis. In the experimental phonon spectra the peaks at 10 and 14 meV are dominated by Na dynamics that disappear on warming, suggesting low-energy phonons significantly contribute to large Na vibrational amplitude at elevated temperatures that enhances the Na hopping probability. We have also calculated the mode Gr\"uneisen parameters of the phonons and thereby calculated the volume thermal expansion coefficient, which is found to be in excellent agreement with available experimental data.