Investigation of the polarization dependence in the XANES spectra and of the specific heat of NaxCoO2

• Main Authors: Chia-Chih Hsu, 徐家治
• Other Authors: Jiunn-Yuan Lin
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/21805945507790388558

碩士 === 國立交通大學 === 物理研究所 === 96 === The soft X-ray absorption spectra (XAS) of NaxCoO2 revealed marked and puzzling polarization dependence. It can not be explained by the degeneracy of eg states generally believed in NaxCoO2. We fabricated the thin films of x = 0.68 and x = 0.75 to investigate the polarization dependence of XAS. We also try to explain this phenomenon with the first principle DFT calculations. After the analysis of the DOS of NaxCoO2, we propose that the pre-edge peaks at 529 eV and 530 eV of NaxCoO2 O-K edge may be not solely due to the unoccupied states of Co3+ and Co4+ eg states, but also due to the spacial asymmetry in theoccupied Co 3d orbitals. Due to the hybridization between Co 3d & O 2p orbitals, the px,y and pz states will be non-degenerate. Besides the XANES measurements, we performed the specific heat and magnetization experiments on Na0.832CoO2 single crystal to investigate the phase diagram. It is found that there are two new phase transitions at T = 8 K or T = 15 K, respectively. The occurrence of the phase transition at T = 15 K and T = 8 K depend on the cooling rate through the temperature region that the mobility of Na ions exists. Finally, we conclude that both the 8 K and 15 K transitions are of the antiferromagnetic ordering at zero field. The transition from antiferromagnetism to ferromagnetism was observed for both ordering states. Therefore, metamagnetism was observed in both 8 K and 15 K transitions of Na0.832CoO2.