Effects of water intercalation in Na _x CoO _2y H ₂ O

Water intercalation is necessary for the occurrence of superconductivity in Na _x CoO _{2 y} H ₂ O. It is known that water insertion leads to the elongation of the c-axis. Beyond that, it remains controversial about the effects of water intercalation on the electronic structure of Na _x CoO _{2 y} H ₂ O. In this paper, we report the Co-K edge spectroscopy on y=0.7 and 1.4 samples, respectively. The spectra are compared with those of the reference samples. It is found that the variation of the Co valence can not be explained by the simple electron counting. These results are consistent with those of the previous chemical titration.