Electronic and structural studies of transition metal oxides (PhD)

Abstract

Interest in transition metal oxides has revived since the discovery of high Tc-superconductivity and colossal magnetoresistance. The observations of intrinsic inhomogenities in these systems have revolutionized the understanding of the complex phase diagram. The present thesis is focused on two different classes of materials namely perovskite based La0.2Sr0.8MnO3 and quasi one dimensional Sr3NiRhO6 compounds. The thesis is aimed at understanding the fundamental issues associated with both class of compounds using synchrotron based high resolution x-ray diffraction technique, inverse photoemission technique and synchrotron based x-ray absorption spectroscopy. Our results show that La0.2Sr0.8MnO3 is structurally inhomogeneous in the nanometer scale for a wide temperature region which is an intrinsic property of the system. The coexisting phases are tetragonal which is orbital ordered, insulating and cubic which is charge ordered. From the results, it appears the possibility of metallicity associated with the cubic charge ordered state. In the insulating phase, we observed unusually an increased intensity at around the Fermi level in the low temperature inverse photoemission spectra. This puzzling behaviour is attributed to the shift in the chemical potential towards the conduction band and localisation of disorder induced states closer to the conduction band. In the case of quasi one dimensional, Sr3NiRhO6 compound, using Ni and Sr K-edge synchrotron x-ray absorption spectroscopy, we have observed common behaviours associated with isostructural compounds. There occurs onset of intra-chain and inter-chain magnetic interactions at temperatures much above the transition temperature and also the role played by direct d-d hybridization is less significant in determining the magnetic interactions. The thesis is structured into 6 chapters. The first chapter gives details about the basic physics that is required to understand the materials under study. The chapter 2 deals with the details of the experimental techniques and computational tool that have been used for the data analysis. Chapters 3&4 discuss the results of the temperature-dependent high-resolution x-ray diffraction experiments and inverse photoemission experiments carried out on La0.2Sr0.8MnO3. Chapter 5 discusses the results of the temperature-dependent x-ray absorption experiments carried out on quasi one dimensional Sr3NiRhO6 compound. In Chapter 6, we summaries the results and discuss the future studies that can be done on the systems that have been studied in the thesis.