| Summary: | 碩士 === 國立交通大學 === 物理研究所 === 102 === In this thesis, X-ray absorption near edge spectroscopy (XANES) was employed to study the valence and spin degree of freedom of transition metal ions in materials.
For the case of Ni(NO3)2, the Ni L-edge spectrum indicates divalence nickel in Ni(NO3)2 compounds. Combined with the theoretical calculations, we confirm that the divalent nickel exhibits the high spin state (S=1) in Ni(NO3)2. In the study, NiO (Ni2+, S=1) was used as the standard sample. The spectrum shape differences between NiO and Ni(NO3)2 come from the different local environments.
Moreover, we measured room temperature Mn K, L2,3-edge and Co K, L2,3-edge and temperature dependent Co L2,3-edge XANES spectra in PrSrCoMnO6 double perovskites. Our spectra show the Co3+/Mn4+ ionic state in PrSrCoMnO6. With the help of theoretical calculations, we found that the crystal field in CoO6 octahedral is close to the HS-LS transition boundary. The crystal structure in PrSrCoMnO6 was solved at T=300 K and T=12 K, respectively. PrSrCoMnO6 shows an unusual lattice change at low temperatures. The lattice volume and the average Mn-O bond length decrease, but the average Co-O bond length increases at low temperatures. We also find there is a close relationship between CoO6 local environment and Co L-edge spectra line shape. The temperature dependent Co L-edge XAS spectra of PrSrCoMnO6 obviously demonstrate the spin state variation with T, which can be described as the combination of a LS ground state and the triply degenerate HS first excited states.
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