DIFFRACTION STUDIES ON STRONGLY CORRELATED PEROVSKITE OXIDES

In recent years, a great interest has been devoted to the so called strongly correlated systems containing perovskite building blocks. These systems exhibit a complex interplay between charge, spin, orbital and lattice degrees of freedom paving the way for very attractive applications. In this work, entitled “Diffraction Studies on Strongly Correlated Perovskite Oxides” the use of x-ray diffraction techniques to investigate the coupling between the structure and the physical properties of several bulk material based on perovskite structure is presented. The thesis is organized in five chapters. Introduction presents a very general overview on strongly correlated perovskite oxides and the scope of the thesis. The first chapter reports technical details of the diffraction techniques involved in all the structural studies performed during the PhD. Chapter 2 reports an accurate investigation performed on the magnetoresistive cobaltite GdBaCo2O5+d (d=0) using single-crystal and synchrotron powder X-ray diffraction. In this work, we assign the correct space group and we demonstrate that a very small tetragonal-to-orthorhombic lattice distortion is coupled to magnetic phase transition. In Chapter 3, we show the study of the temperature induced insulator-to-metal transition for GdBaCo2O5+d (d>0.5). By using a combined approach between electron paramagnetic resonance and powder diffraction techniques we provide new interesting features about the spin – lattice interaction occurring in these systems. Chapter 4 presents synchrotron X-ray powder diffraction study on EuTiO3 system. We show for the first time the existence of a new structural phase transition occurring in EuTiO3 below room temperature. In addition, by performing the atomic pair distribution function analysis of the powder diffraction data, we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material and we propose that the lattice disorder is of fundamental importance to understand the EuTiO3 properties. Finally, beyond the scope of the thesis, in Chapter 5, we review the basic procedure to get the differential pair distribution function obtained by applying the anomalous X-ray diffraction technique to total X- ray scattering method. We show an example of the application of this procedure by presenting the case of gadolinium doped ceria electrolytes. This work will show that use of the powder diffraction techniques provides a powerful tool to unveil the coupling between the structure and the physical properties in strongly correlated perovskite oxides.