| The employed experimental method in this Ph.D. dissertation research is the atomic pair distribution function (PDF) technique specializing in high real space resolution local structure determination. The PDF is obtained via Fourier transform from powder total scattering data including the important local structural information in the diffuse scattering intensities underneath, and in-between, the Bragg peaks. Having long been used to study liquids and amorphous materials, the PDF technique has been recently successfully applied to highly crystalline materials owing to the advances in modern X-ray and neutron sources and computing power. An integral part of this thesis work has been to make the PDF technique accessible to a wider scientific community. We have recently developed the rapid acquisition PDF (RA-PDF) method featuring high energy X-rays coupled with an image plate area detector, allowing three to four orders of magnitude decrease of data collection time. Correspondingly in software development, I have written a complete X-ray data correction program PDFgetX2 (user friendly with GUI, 32,000+ lines). Those developments sweep away many barriers to the wide-spread application of the PDF technique in complex materials. The RA-PDF development also opens up new fields of research such as time-resolved studies, pump-probe measurements and so on, where the PDF analysis can provide unique insights. Two examples of the RA-PDF applications are described: the distorted T12 square nets in the new binary antimonide Ti2Sb and in-situ chemical reduction of CuO to Cu.; The most intellectually enriching has been the local structure studies of the colossal magneto-resistive (CMR) manganites with intrinsic inhomogeneities. The strong coupling between electron, spin, orbital, and lattice degrees of freedom result in extremely rich and interesting phase diagrams. We have carried out careful PDF analysis of neutron powder diffraction data to study the local MnO6 octahedral distortions. For example, in the updoped compound LaMnO3, the Jahn-Teller (JT) transition around 750 K is characterized as distorted (JT active) to undistorted MnO6 octahedra transition by the conventional crystallographic analysis. However, our PDF results show local MnO6 octahedral distortions persist above the JT transition, and it is their random orientations that make the existing local distortions invisible to average structure analysis. The nature of the JT transition around 750 K is orbital order to disorder. Our first local structure study of the high temperature rhombohedral phase (T ≥ 1010 K) additionally discovered the existence of locally JT distorted MnO6 octahedra. More significantly, the range of the orbital order in the high temperature dynamic-JT phases; estimated from the crossover from the local to the average structure, is around 16 A (∼four MnO6 octahedra), suggesting strong nearest neighbor JT anti-ferrodistortive coupling. In the bi-layered La0.92Sr 2.08Mn2O7, we found the shape of MnO6 octahedron changes from oblate (4 long, 2 short Mn-O bonds) to prolate (2 long, 4 short Mn-O bonds) as the material goes from type A anti-ferromagnetic to type CE charge ordered phase. This can be understood as the d x2-y2 to dx,y2-r2 orbital occupancy transition of the Mn3+ e g electrons. Evidences for nano-scale inhomogeneities in the Mn4+ rich region of La2-2xSr 1+2xMn2O7 (0.54 ≤ x ≤ 0.80) are also discussed. |
