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.
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