A molecular dynamics simulation study of ionic diffusion and NMR spin–lattice relaxation in Li 2Si 4O 9 glass

Abstract

The effect of ionic diffusion on Li spin–lattice relaxation (SLR) in Li 2Si 4O 9 glass has been studied using the molecular dynamics simulation technique over the temperature range 2000–3800 K. The orientational pair correlation function G( t) associated with SLR has been computed for individual Li ions from the trajectories of the constituent ions at different temperatures. The temporal decay of G( t) is found to be non-exponential and can be described by the stretched-exponential functional form exp(− t/ τ SLR) β . It is shown that the non-exponential behavior of G( t) is intrinsic to the system, possibly originating from hierarchically constrained relaxation processes. The activation energies associated with Li diffusion, τ SLR and the average conductivity relaxation time 〈 τ σ 〉 are shown to be in excellent agreement indicating that the same microscopic dynamic processes are responsible for SLR, diffusion and conductivity. However, τ SLR is found to be nearly an order of magnitude longer than 〈 τ σ 〉 over the entire temperature range of these simulations. This result reflects the fundamental differences between the correlation functions associated with SLR and conductivity relaxation. The existing experimental data in the literature on SLR in normal and fast-ion conducting glasses are shown to be completely consistent with the results obtained from the present simulations.