  We have investigated the dynamics of Na ions in amorphous Na2Si2O5, a
potential solid electrolyte material for Na-battery. We have employed
quasielastic neutron scattering (QENS) technique in the amorphous Na2Si2O5 from
300 to 748 K to understand the diffusion pathways and relaxation timescales of
Na atom dynamics. The microscopic analysis of the QENS data has been performed
using ab-initio and classical molecular dynamics simulations (MD) to understand
the Na-ion diffusion in the amorphous phase. Our experimental studies show that
the traditional model, such as the Hall and Ross (H-R) model, fairly well
describe the diffusion in the amorphous phase giving a mean jump length of ~3
{\AA} and residence time about 9.1 picoseconds. Our MD simulations have
indicated that the diffusion of Na+ ions occurs in the amorphous phase of
Na2Si2O5 while that is not observed in the crystalline orthorhombic phase even
up to 1100 K. The MD simulations have revealed that in the amorphous phase, due
to different orientations of silicon polyhedral units, accessible pathways are
opened up for Na+ diffusions. These pathways are not available in the
crystalline phase of Na2Si2O5 due to rigid spatial arrangement of silicon
polyhedral units.
