O'3-Na(x)VO2 system: a superstructure for Na(1/2)VO2.

The electrochemical cycling in a sodium battery of the lamellar oxide NaVO(2) is reversible in the Na(x)VO(2) composition range 1/2 ≤ x ≤ 1. The complex electrochemical curve reveals the presence of several transitions taking place during deintercalation. With the help of in situ X-ray diffraction, we observed the structural transitions taking place between Na(2/3)VO(2) and Na(1/2)VO(2). The diffractograms show the presence of several monophasic domains separated by biphasic domains. All phases present a monoclinic distortion of the α-NaFeO(2) structure in the composition range 1/2 ≤ x ≤ 2/3. Moreover the presence of a superstructure is evidenced for Na(1/2)VO(2). It is the first time that an ordered structure is reported at the Na(1/2)MO(2) composition with an O'3 oxygen stacking. A thorough investigation of electrochemically obtained O'3-Na(1/2)VO(2) was performed. The structure refinement reveals the existence of a sodium/vacancy ordering, with a peculiar arrangement of the V-V distances hinting at a pairing of vanadium atoms. Our first measurements of the physical properties of O'3-Na(1/2)VO(2) show a semiconductor behavior and a complex thermal dependence of the magnetic susceptibility related to the pairing of the vanadium atoms.