Structural transition in NaxCoO2 with x near 0.75 due to Na rearrangement -: art. no. 134115

We report neutron powder diffraction measurements on a series of NaxCoO2 samples with x near 0.75 which were prepared under different synthesis conditions. Two different crystal structures for the samples are observed at room temperature. The two structures belong to the space group P6(3)/mmc and differ primarily by a shift of a large fraction of the Na ions from a high-symmetry position to a lower-symmetry position. Close inspection of the refinement parameters indicates that the presence of either structure depends sensitively on the Na content x, with xsimilar or equal to0.75 as the critical concentration separating the two phases. By raising the temperature to around Tsimilar or equal to323 K, the high-symmetry structure can be converted to the lower-symmetry structure. The transition is reversible, but there is significant hysteresis. We discuss the effects of this structural transition on the bulk magnetic and transport properties.