Precise control of the Na nonstoichiometry in Na$_{x}$CoO$_{2}$

Na$_{x}$CoO$_{2}$ has been attracting great interests in terms of a correlation between its electronic properties and Na nonstoichiometry. Many groups have reported that a Na rich phase ($x \quad \sim $ 0.7) is a Curie-Weiss metal and a poor phase ($x \quad \sim $ 0.3) is a Pauli paramagnetic metal. The origin of this difference, however, has not been confirmed yet, mainly because of difficulty in controlling precisely the Na nonstoichiometry. We succeeded in synthesizing a series of polycrystalline samples of Na$_{x}$CoO$_{2}$ with well-controlled Na content by the solid-state reaction instead of the solution reaction previously used. We prepared polycrystalline Na$_{x}$CoO$_{2}$ (0.58 $\le \quad x \quad \le $ 0.63) by a solid-state reaction of Na$_{0.71}$CoO$_{2}$ and Na$_{0.5}$CoO$_{2}$ at 200$^{\circ}$C. Furthermore, fine tuning of Na content in the range of 0.62 $< \quad x \quad <$ 0.63 was carried out by a solid-state reaction of Na$_{0.62}$CoO$_{2}$ and Na$_{0.63}$CoO$_{2}$. Magnetic susceptibility of Na$_{x}$CoO$_{2}$ exhibited Curie-Weiss behavior at $x \quad \ge $ 0.621 while nearly temperature independent paramagnetism at $x \quad \le $ 0.620. Such a drastic change of magnetism clearly indicates that the magnetic phase boundary is located in an extremely narrow range around $x$ = 0.62. We think that the difference originates from a change of the Fermi surface topology with electron filling probably associated with the dip in the a$_{1g}$ band near the $\Gamma $ point.