Diffrazione elettronica 3D di Materiali Funzionali Complessi

Abstract

The enduring challenge in modern materials science is to disclose the relationship between materials structure and properties. The design of materials with improved efficiency in exploiting the intrinsic properties is one of the most important purposes of a new generation of smart materials. Multiferroic compounds, perovskites with giant magnetoresistance effect, magnetoelectric materials, ionic conductors and magnetic hybrid organic-inorganic porous frameworks are often characterized by the mutual interactions of physical and chemical properties. Hence, the structural characterization, being physical and chemical properties intrinsically related to the crystal structure, represents a fundamental aspect in this scenario. However, these kinds of materials are often characterized by complex structures. The development of innovative tools for the structural investigation of materials is continuously growing and new diffraction techniques have been implemented in the last decades. In this context the novel approach based on the collection of 3D electron diffraction (ED) data is revolutionizing the structural analysis of nanocrystalline and complex materials. The classical structural information retrieved from a transmission electron microscope (TEM) was essentially two-dimensional, with data derived by zone axis diffraction patterns. The recent development of novel technique and protocols of data collection implemented on a conventional TEM allows to convert the instrument as a single-crystal diffractometer. The principal limitations in the application of ED for the crystal structure determination are derived by strong dynamical effects that suppress the relations between structure factors and reflections intensity in the reciprocal space. This fundamental drawback was tackled by introducing two experimental solutions represented by the precession of primary electron beam3 and the reciprocal space collection through the automated sequence of non-oriented patterns, i.e. electron diffraction tomography (ADT)4. The combined use of these two techniques provides quasi-kinematical ED intensities that were adopted to solve crystal structures in several fields. In Figure 1 it is schematically indicated the different materials whose crystal structures was successfully solved by the application of the 3D ED analysis specially in absence of single crystals suitable for the classical X-ray diffractometry. In this rapidly evolving context, the main purpose of this thesis is represented by the study of the fundamental concepts supporting the electron crystallography and by the application of this novel structural analysis to study complex crystals structure featuring different types of materials.