Dynamics across the structural transitions at elevated temperatures in Na0.7CoO2
EPJ Web of Conferences
The layered transition-metal oxide Na x CoO 2 has been studied extensively both for its correlated electronic properties as well as for potential battery applications. It was discovered that high-temperature Na ion vacancy order and dynamics can be very useful to tailor low-temperature properties of members of this compound family. We have studied the Na-ion dynamics on the atomic length-scale in the Na 0.7 CoO 2 compound by neutron spectroscopy. The temperature dependence of both the elastic
... both the elastic and the inelastic intensities show steps at T A ≈ 290 K and T B ≈ 400 K. At T A the step is shown to be connected to low energy phonons, while at T B the Na ion diffusion suddenly gets fast enough, and the characteristic signal of quasielastic scattering appears. The current results further elucidate the subtle changes in the Na ion dynamics that have been revealed in our previous neutron diffraction studies  , intimately connecting structural transformations at T A and T B with the opening-up of 1D and 2D Na-ion diffusion paths. Finally, the estimated diffusion coefficient above T B was found to differ from the one measured by muon-spin relaxation (µ + SR)  by about four orders of magnitude. However it might be that the present QENS data rather describe a fast localized prozess than a long range translational diffusion. Within this model the corresponding time scale ( /E) would be in the order of 50 ps. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.