  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.
