  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 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 x =
0.75 as the critical concentration separating the two phases. By raising the
temperature to around T = 323 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.
