| Summary: | 碩士 === 國立交通大學 === 物理研究所 === 97 === Recently, there have been intensive research activities on NaxCoO2. However, for the phase diagram of NaxCoO2, especially when x > 0.75, there are still puzzles due to the inadequate sodium content control. This thesis compared the differences of single-crystalline Na0.82CoO2 and Na0.833CoO2 by low temperature heat capacity experiments and magnetic measurments. Na0.833CoO2 have antiferromagnetic transitions at T = 15 K and T = 8 K, respecting Na0.82CoO2 has an antiferromagnetic transition at T = 22 K, and an anomaly around T = 6 K. This study intends to further complete the phase diagram of NaxCoO2.
We have also measured the magnetic susceptibility of single crystal Na0.833CoO2, and point out that there was an unknown transition aroud T = 50 K. We discusse the differences of the easy axis of the three samples: Na0.82CoO2, Na0.832CoO2 and Na0.833CoO2, and tried to understand the effect made by the 1% Sodium content difference in NaxCoO2.
Ca3Co4O9 has also been an interesting compound duo to its thermoelectric properties. Most previous studies focused on the room temperature properties, and rarely discussed those at low temperature. We measure bulk Ca3Co4O9 by heat capacity and magnetic susceptibility experiments at low temperatures. The magnetic susceptibility behaves like a ferrimagnetice, but the specific heat data do not indicate a short range ordering. The 16 K anomaly of Ca3Co4O9 in specific heat disappears when we for Bi-doped Ca3Co4O9.
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