Stoichiometric Tuning of Lattice Flexibility and Na Diffusion in NaAlSiO4: Quasielastic Neutron Scattering Experiment and Ab-initio Molecular Dynamics Simulations

TL;DR

This study combines neutron scattering experiments and molecular dynamics simulations to explore how lattice flexibility influences sodium diffusion in different phases of NaAlSiO4, revealing phase-dependent diffusion behaviors and the role of framework dynamics.

Contribution

It introduces a combined experimental and computational approach to elucidate Na diffusion mechanisms and the impact of lattice flexibility in NaAlSiO4 phases.

Findings

Na ions show localized diffusion in L-NASO and N-NASO

Long-range Na diffusion occurs in H-NASO due to framework flexibility

Excess Na enhances host network flexibility and activates tetrahedral rotations

Abstract

We have performed quasielastic neutron scattering (QENS) experiments up to 1243 K and ab-initio molecular dynamics (AIMD) simulations to investigate the Na diffusion in various phases of NaAlSiO4 (NASO), namely, low-carnegieite (L-NASO; trigonal), high-carnegieite (H-NASO; cubic) and nepheline (N-NASO; hexagonal) phases. The QENS measurements reveal Na ions localized diffusion behavior in L-NASO and N-NASO, but long-range diffusion behavior in H-NASO. The AIMD simulation supplemented the QENS measurements and showed that excess Na ions in H-NASO enhance the host network flexibility and activate the AlO4/SiO4 tetrahedra rotational modes. These framework modes enable the long-range diffusion of Na across a pathway of interstitial sites. The simulations also show Na diffusion in Na-deficient N-NASO through vacant Na sites along the hexagonal c-axis.