Correlation between Tc and Lattice Parameters of Novel Superconducting   NaxCoO2 yH2O
  We synthesized the five batches of the samples of the novel P3 type superconductor, Na$_{x}$(H$_{3}$O)$_{y'}$CoO$_{2}\cdot y''$H$_{2}$O, by the soft chemical process starting from $\alpha$-NaCoO$_{2}$. The chemical and structural properties varied rather widely from batch to batch, with a result that $T_{c}$ varied from 4.6 K to 3.2 K. The magnetic susceptibility above $T_{c}$ shows upturn at low temperature as in the case of the P2 phase. The $T_{c}$ seems to be well correlated to the lattice parameters. 
H. Sakurai, K. Takada, T. Sasaki, F. Izumi, R. A. Dilanian, and E.   Takayama-Muromachi