Precise Control of Na Content in the Layered Cobaltate γ-Na x CoO2

We have developed a preparation method for precisely controlling the Na content x of a gamma-Na0.7CoO2 precursor by a halogen oxidation technique in the range 0.50 <= x <= 0.70. The Na content of the precursor was reduced in a concentration-controlled I-2-CH3CN solution. The magnetic susceptibility of gamma-NaxCoO2 shows Curie-Weiss (CW)-type paramagnetism ( x approximate to 0.70) or Pauli-type paramagnetism ( x approximate to 0.50). The boundary of the CW and Pauli paramagnetic phases was identified at x approximate to 0.61. The content and temperature for a disorder-order transition from the c phase to a root 7a(0) x root 7a(0) superstructure were accurately elucidated by differential scanning calorimetry (DSC) measurement. A phase transition for the 0.52 <= x <= 0.54 sample was observed at 250 K by DSC measurements. The heat absorption of the 0.52 <= x <= 0.54 samples was of the same order of magnitude as that of the transition from the gamma phase to the root 7a(0) x root 7a(0) phase