Duration dependences of electric and crystal structures of bi layer hydrated NaxCoO2·yH2O

We succeeded in synthesizing the bi layer hydrated superconducting NaxCoO2·yH2O powder samples with different Tc's by systematically changing the duration in a high humidity atmosphere after intercalation of water molecules. From the results of 59Co nuclear quadrupole resonance measurements, it was found that resonance frequency νQ monotonically decreased with increasing the duration after filtration. The electronic states of samples were found being well characterized by the νQ, and we succeeded in drawing the electronic phase diagram in which there exist two superconducting phases and an in-between magnetic phase. On the other hand, a lattice parameter along c-axis was found to decrease monotonically with increasing the time duration. This result indicates that shrinkage of CoO2 planes is caused by changes of arrangements and/or contents of ions between CoO2 planes which evolve during kept in a high humidity atmosphere.