  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.
