  Na-ion conduction, and correlations between Na-ion conduction pathways and
crystal structure have been investigated as a function of temperature in the
layered battery material Na2Ni2TeO6 by impedance spectroscopy and neutron
diffraction, respectively. The impedance data reveal an ionic conductivity
2x10-4 S/m at 323 K which strongly enhances with increasing temperature and
attains a high value of ~ 0.03 S/m at 423 K. The temperature dependent
conductivity data show an Arrhenius type behavior with average activation
energy (Ea) of ~ 0.58(3) eV for T > 383 K. By employing soft-bond valence sum
analysis of the neutron diffraction patterns we experimentally demonstrate the
site-specific Na-ion conductions, through visualization of microscopic
sodium-ion conduction pathways, and verify the recent theoretical results of
Molecular dynamic simulation. Our results reveal two-dimensional Na-ion
conduction pathways that are confined within the ab planes of Na layers.
Crystal structural study indicates that the layered structure involving Na ion
layers is responsible for high ionic conductivity, and the local
crystallographic environment of Na ion sites is responsible for site-specific
conductivity. Our study further reveals that up to 500 K, the ionic conduction
is governed by the Na ions located at the Na1 and Na2 sites, whereas, all the
Na ions located at the three Na-sites contribute to the conduction process
above 500 K. Our neutron diffraction study also establishes that the crystal
structure of Na2Ni2TeO6 is stable at least up to 725 K (the highest measured
temperature), however, with an anisotropic thermal expansion (c/a ~ 3).
