Abstract
We have developed a preparation method for precisely controlling the Na content x of a γ-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 I2-CH3CN solution. The magnetic susceptibility of γ-Na x CoO2 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 \(\sqrt{7}a_0 \times \sqrt{7}a_0\) 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 \(\sqrt{7}a_0 \times \sqrt{7}a_0\) phase.
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Acknowledgements
We thank Mr. K. Nemoto for providing technical assistance in the HERMES experiment. This study was partly supported by the Core Research for Evolution Science and Technology (CREST) Project of the Japan Science and Technology Agency (JST), and also by a Grant-in-Aid for the Global COE Program “International Center of Research and Education for Molecular Complex Chemistry (iremc)” from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.
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Igarashi, D., Miyazaki, Y., Yubuta, K. et al. Precise Control of Na Content in the Layered Cobaltate γ-Na x CoO2 . J. Electron. Mater. 39, 1669–1673 (2010). https://doi.org/10.1007/s11664-010-1238-9
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DOI: https://doi.org/10.1007/s11664-010-1238-9