Stoichiometric tuning of lattice flexibility and Na diffusion in NaAlSiO<inf>4</inf>: quasielastic neutron scattering experiment andab initiomolecular dynamics simulations

We have performed quasielastic neutron scattering (QENS) experiments up to 1243 K andab initiomolecular 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/SiO4tetrahedra 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 hexagonalc-axis.