  A systematic study of NaxCoO2 (x=0.50 - 0.90) and NaxCoO2 yH(D)2O (x=0.26 -
0.42, y=1.3) has been performed to determine phase stability and the effect of
hydration on the structural and superconducting properties of this system. We
show that a careful control of the Na deintercalation process and hydration
dynamics is possible in single crystals of this system. Furthermore, we give
experimental evidence that the dependence of the superconducting transition
temperature on Na content is much weaker than reported earlier. Implications of
this effect for the understanding of the superconducting phase diagram are
discussed.
