TbMnO3的電子結構與DyMnO3及Ca3CoMnO6的高壓之研究

Abstract

本論文的內容主要是針對多鐵TbMnO3的電子結構與DyMnO3, Ca3CoMnO6外壓效應之研究，論文的第一個部分是探討多鐵TbMnO3的鍵結之異向性與軌道的對稱性，我們完成TbMnO3的軟光吸收光譜和共振非彈性X散射的測量，結合電子密度泛函理論及電子結構交互作用之叢集計算，根據實驗和理論計算的結果，我們解釋說明了多鐵TbMnO3的異向性電子結構。 論文的第二個部分是研究多鐵DyMnO3與Ca3CoMnO6在高壓下壓的效應，根據X光繞射數據的結果，觀察到高壓抑止了DyMnO3內部Mn的八面體MnO6的Jahn-Teller扭曲，由我們光譜也證實了壓力引起Mn的電子能帶變寬之現象，同時比較Mn K-edge X光吸收光譜與電子結構及FDMNES的全多重散射計算得到一致結果。另外在Ca3CoMnO6的研究方面，我們完成了X光繞射、X光吸收光譜和X光放射光譜等加壓到~45 GPa之量測，我們利用積分絕對差值(IAD)法分析Ca3CoMnO6中Co3+的自旋狀態，依據X光繞射、X光吸收光譜和X光放射光譜的實驗結果，我們證實了在高壓下Ca3CoMnO6中由高自旋到低自旋的自旋轉變。此外由1s3p的共振X光放射光譜我們觀察到在多鐵Ca3CoMnO6的Co K-edge光譜中，發現在拉曼區域的1s到3d被抑止的四極矩躍遷中出現的小峰，顯示出Co 3d的非佔據態相對非局域化特徵，進一步分析Co Kβ共振X光放射光譜，我們發現這遠距的偶極Co 2p到3d的躍遷，是源自於非局域Co 4p與相鄰Co 3d混成態所產生的結果。

The aim of this thesis is to investigate the electronic structure of multiferroic TbMnO3 and external pressure effect of DyMnO3 and Ca3CoMnO6. First part of this thesis is to investigate the anisotropic bonding and orbital symmetry of TbMnO3. We performed measurements of soft x-ray absorption (XAS) and resonant inelastic x-ray scattering, coupled with electron density-functional theory and configuration-interaction cluster calculations in TbMnO3. Based on the XRD results of experiment and calculation, we elucidate anisotropic electronic structure of multiferroic TbMnO3. Second part of the thesis is to investigate high pressure effect in multiferroics DyMnO3 and Ca3CoMnO6. Based on X-ray diffraction (XRD) results, a local Jahn-Teller distortion of Mn sites within MnO6 octahedra in DyMnO3 becomes suppressed with increasing pressure. We provide spectral evidence for the pressure-induced band width broadening for manganites. The comparison of XAS data with ab initio electronic structure calculations and full multiple-scattering calculations using code FDMNES shows satisfactory agreement between experimental and calculated Mn K-edge spectra. In Ca3CoMnO6, we carry out XRD, XAS, and X-ray emission spectroscopy (XES) experiments under pressure up to ~ 45 GPa. We analyzed the spin state of Co3+ in Ca3CoMnO6 using integrated absolute difference (IAD) method. Based on the analysis on XRD, XAS and XES, we evidenced for a spin-state transition from the HS to LS transition under pressure in Ca3CoMnO6. The 1s3p-RXES spectra obtained in multiferroic Ca3CoMnO6 at the Co K edge reveal that the quadrupolar Raman regime is restricted predominantly to below the pre-edge peak whereas the fluorescence regime starts from the first pre-edge peak of 1s→3d transitions, indicating a relatively delocalized character of unoccupied Co 3d states. In additional Kβ emission profile is proposed to be attributed to the contribution of the off-site dipole Co 1s-Co’ 3d transitions, as a consequence of hybridization between Co 4p states and neighboring Co’ 3d states.