Dynamics across the structural transitions at elevated temperatures in Na_0.7CoO₂

¹ Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, Villigen PSI, Switzerland
² Laboratory for Quantum Magnetism, EPF Lausanne, Switzerland
³ Laboratory for Development and Methods, Paul Scherrer Institute, Villigen PSI, Switzerland
⁴ Toyota Central Research and Development Laboratories, Inc., Nagakute, Aichi, Japan
⁵ Comprehensive Research Organisation for Science and Society, Tokai, Ibaraki, Japan
⁶ Laboratory for Solid State Physics, ETH Zürich, Zürich, Switzerland
⁷ Institut Laue-Langevin, Grenoble, France

^a e-mail: fanni.juranyi@psi.ch

Published online: 23 January 2015

Abstract

The layered transition-metal oxide Na_x CoO₂ 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 CoO₂ compound by neutron spectroscopy. The temperature dependence of 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 [1], 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) [2] 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 (ℏ/E) would be in the order of 50 ps.