- 學位論文
研究高溫高壓下多鐵材料YMnO3之晶體和電子結構以及研究鐵摻雜YMnO3單晶薄膜之結構
Study of electronic and crystal structures in multiferroic YMnO3 perovskite as a function of high temperature/pressure and in Fe-substituted YMnO3 epitaxial film on YAlO3(010) substrate
摘要
本論文主要內容以同步輻射光源為基礎並搭配相關技術，如X光繞射光譜，以K-edge的高解析X光吸收光譜和1s3p共振放射光譜針對過渡金屬元素錳和鐵離子，來進行研究具鈣鈦礦結構之多鐵材料釔錳氧多晶體在高溫和高壓下的晶體和電子結構。根據X光繞射光譜，釔錳氧在高溫和高壓下沒有結構相變。我們發現在高溫下楊泰勒晶格形變的減小，主要是由錳氧錳鍵角變大或八面體MnO6間的扭曲減小所造成，而在高歷下楊泰勒晶格形變的減小，則是由八面體MnO6中的錳氧長鍵被壓縮所影響。在高溫及高壓下楊泰勒晶格形變的減小，會使得兩相隣錳離子間的4p‒3d 軌道混成變大。我們亦發現釔錳氧在高溫或高壓下會有金屬行為的發生。 此外，我們籍由過渡金屬元素錳和鐵離子在其K-edge和L2,3-edge的極化相關X光吸收光譜，研究純釔錳氧及鐵離子取代之釔錳氧單晶薄膜中的楊泰勒晶格形變及其電子組態。在理論計算方面，我們則分別利用FDMNES全多重散射計算來進行模擬過渡金屬元素錳離子在其K-edge的極化相關X光吸收光譜，以及利用configuration-interaction 多重散射計算來進行模擬過渡金屬元素錳離子在其L2,3-edge的極化相關X光吸收光譜。根據XRD結果，Fe3+離子取代Mn3+離子對晶格常數所產生的影響不大。當50% Mn3+離子被 Fe3+離子取代時，其Δeg值因受到球形對稱的Fe3+晶體場的影響會下降至0.6 eV (Mn3+離子在純釔錳氧的Δeg為 0.9 eV)，但eg電子仍佔據在d3y2-r2軌道。另一方面，Fe3+離子的Δeg值則會受到Mn3+離子的影響，會從 0.15 eV (Fe3+離子的濃度為12.5%)上升至 0.5 eV (Fe3+離子的濃度為50%)，且Fe3+離子會因對稱性極差之Mn3+離子晶體場的效應而產生相同的扭曲變形。從極化相關吸收光譜及理論計算所得的結果，我們推測在50% Fe3+離子取代的樣品中的磁結構屬 G-type 結構。
關鍵字
多鐵材料並列摘要
The crystal and electronic structure of multiferroic o-YMnO3 polycrystals prepared under high pressure and temperature were investigated as a function of temperature up to 873 K and external pressure up to 35 GPa by synchrotron x-ray diffraction (SXRD), high-resolution x-ray absorption spectra (XAS) at Mn K-edge, 1s3p resonant x-ray emission spectra (RXES) at Mn K-edge, and Mn K-edge XAS with full multiplet-scattering calculations using FDMNES code. No phase transition in metastable o-YMnO3 up to pressure 25 GPa or temperature 873 K was observed. The Jahn-Teller (JT) reduction at high temperature is mainly caused by the increasing Mn-O-Mn bond angle or the reduction of MnO6 octahedra distortion, whereas JT reduction at high pressure is dominated by the shortening of long MnO bond in MnO6 octahedra. The reduction of JT distortion at high temperature or pressure will result in larger oxygen-mediated Mn 4p‒Mnʹ 3d intersite hybridization and thus will have a significant influence on the physical properties of o-YMnO3. Moreover, a temperature- or pressure-induced metallic behavior is enclosed in o-YMnO3. In the last part of the thesis, Fe-substituted o-YMn1-xFexO3(020) (for x=0.125, 0.25, 0.50) epitaxial thin films on substrate YAlO3(010) were investigated by the measurements of single crystal XRD, magnetization as a function of temperature and magnetic field, the polarization-dependent XAS at the O K-edge with LDA+U calculations, and the polarization-dependent XAS at the Mn-L2,3 and Fe-L2,3 edges with the configuration-interaction multiplet calculations. Upon Fe substitution, the b-axis is clearly decreased, while a- and c-axis are slightly increased. It is found that although Mn3+-Δeg is significantly reduced from 0.9 eV in pure o-YMnO3 to 0.6 eV in the half Fe-substituted film, a single eg electron is still strongly constrained to d3y2-r2 orbital. On the other hand, the largest Fe3+-Δeg of 0.5 eV is derived for 12.5% Fe concentration and gradually reduced to 0.15 eV in the half Fe-substituted film. Besides, the local octahedral-site distortion of Fe3+ ion inside YMnO3 lattice exhibits the similar way as that in Mn3+ ion, while JT distortion and GdFeO3-type distortion of Mn3+ ions are reduced by the spherical high spin Fe3+ ions. Moreover, the speculation of G-type magnetic structure is favorable in o-YMn0.5Fe0.5O3 thin film. The comprehensive results provide deeper insights of the variation of JT distortion and orbital anisotropy, as well as intriguing magnetic structures in the studied o-YMn1-xFexO3 thin films.