Sodium diffusion and dynamics in Na2Ti3O7: neutron scattering and ab initio simulations

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 and INS measurements were analyzed using ab initio molecular dynamics (AIMD) simulations. From these measurements and simulations, we interpreted that at 1173 K, the diffusion of Na is controlled by the localized jumping motion of Na, while at higher temperature (1800 K) AIMD simulations predicted a long range 1-d diffusion of Na through interstitial sites along the crystallographic a-axis. Furthermore, calculations using the nudged-elastic-band (NEB) method confirmed the lowest activation energy barrier for Na diffusion along the a-axis. In the experimental phonon spectra, the peaks at 10 and 14 meV were dominated by Na dynamics at low temperature, which disappeared on warming, suggesting that low-energy phonons significantly contribute to large Na vibrational amplitude at elevated temperatures that enhance the Na hopping probability. To provide a microscopic understanding of the thermal expansion behaviour and to correlate that with Na dynamics, we have also calculated the mode Grüneisen parameters of the phonons. From this investigation, we have identified that the Na-dominated low-frequency anharmonic phonons strongly contribute to the thermal expansion.