Direct detection of Na⁺ diffusion dynamics in Na₂Ni₂TeO₆

The layered honeycomb compound Na₂Ni₂TeO₆ hosts a framework conducive to both complex magnetic interactions and high ionic conductivity. We report on our studies of Na⁺ diffusion dynamics as probed by neutron backscattering spectroscopy. Na₂Ni₂TeO₆ is a promising sodium battery material and has been reported to exhibit ionic conductivities as high as 10.8 S/m at 300 ºC and 3.4 x 10^-3 S/m at room temperature. Despite their weak interaction with matter, neutrons have a particular advantage in the study of diffusive dynamics in that their quasielastic scattering provides a direct probe on length scales comparable to interatomic spacings and time scales typical for diffusive relaxation times. We have utilized the BASIS instrument at the Spallation Neutron Source to perform QENS studies of Na₂Ni₂TeO₆ in a temperature range of room temperature up to 400 ºC, revealing isotropic diffusion above 250 ºC with an activation energy, E_a = 320 ± 40 meV. Results are discussed in the context of the three non-equivalent sites present in the Na layers.