1D to 2D Na Ion Diffusion Inherently Linked to Structural Transitions in Na$_{0.7}$CoO$_{2}$

TL;DR

This study reveals how structural transitions in Na0.7CoO2 are intrinsically linked to stepwise Na ion diffusion, with implications for tuning material properties via ionic mobility control.

Contribution

It uncovers the connection between structural phase transitions and Na ion diffusion pathways in Na0.7CoO2, providing new insights into ionic dynamics and material tuning.

Findings

Two first-order structural transitions at 290 K and 400 K.

Sequential disappearance of Na-vacancy order enabling 1D and 2D Na diffusion.

Link between structural changes and Na mobility pathways.

Abstract

We report the observation of a stepwise "melting" of the low-temperature Na-vacancy order in the layered transition metal oxide Na0.7CoO2. High-resolution neutron powder diffraction indicates the existence of two first-order structural transitions, one at T1 = 290 K, followed by a second at T2 = 400 K. Detailed analysis reveals that both transitions are linked to changes in the Na mobility. Our data are consistent with a two-step disappearance of Na-vacancy order through the successive opening of first quasi-1D (T1 > T > T2) and then 2D (T > T2) Na diffusion paths. These results shed new light on previous, seemingly incompatible, experimental interpretations regarding the relationship between Na-vacancy order and Na dynamics in this material. They also represent an important step towards the tuning of physical properties and the design of tailored functional materials through an improved control and understanding of ionic diffusion.