1D to 2D Na Ion Diffusion Inherently Linked to Structural Transitions in Na${}_{0.7}$CoO${}_2$

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.

5 pages, 4 figures, accepted by PRL

References in corpus (5)

• Electronic texture of the thermoelectric oxide Na0.75CoO2
• Direct link between low temperature magnetism and high temperature sodium order in NaxCoO2
• Melting of the Na layers in solid Na0.8CoO2
• Local moment, itinerancy and deviation from Fermi liquid behavior in Na${}_x$CoO${}_2$ for $0.71\le x\le 0.84$
• Crystal-to-stripe reordering of sodium ions in NaxCoO2 (x>=0.75)

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