  The structural properties of NaxCoO2 and NaxCoO2yH2O have been investigated.
The NaxCoO2yH2O samples in general show up the superconducting transitions at
around 3.5K. EDAX analyses suggest our samples have the average composition of
Na0.65CoO2 for the parent compound and Na0.26CoO2yH2O for the superconducting
crystals. TEM observation reveals a superstructure with wave vector q=<1/2,0,
0> in the parent Na0.65CoO2 materials. This superstructure becomes very weak in
the superconducting oxyhydrates. EELS analyses show that the Co ions have the
valence state of around +3.3 in the Na0.65CoO2 materials and around +3.7 in the
superconducting materials.
