Precise control of Na content in the layered cobaltate γ-Na _xCoO ₂

We have developed a preparation method for precisely controlling the Na content x of a γ-Na _0.7CoO ₂ 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-CH ₃CN solution. The magnetic susceptibility of γ-Na _x CoO ₂ shows Curie-Weiss (CW)-type paramagnetism (x ≈ 0.70) or Pauli-type paramagnetism (x ≈ 0.50). The boundary of the CW and Pauli paramagnetic phases was identified at x ≈ 0.61. The content and temperature for a disorder-order transition from the γ phase to a √7a ₀ × √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 γ phase to the √7a ₀ × √7a ₀ phase.