Dottorato in MODELLI MATEMATICI PER L'INGEGNERIA, ELETTROMAGNETISMO E
NANOSCIENZE

Titolo della tesi: Study of the functional-local structure relationship of some disordered systems

Disordered systems are encountered in numerous areas of research and have diverse applications in various fields. For instance, disordered materials are widely used in electronic devices, catalysis, and energy conversion, and understanding their structure and behavior is of utmost importance for improving their performance. In condensed matter physics, disordered systems are prevalent, and their properties are essential in understanding the behavior of materials such as amorphous solids and glasses. Disordered systems are also encountered in biological systems such as protein folding, where the local structure is crucial in determining the protein’s function. Disordered systems can exhibit intriguing properties such as localization, where a particle’s motion is constrained to a small region, leading to unusual diffusion behavior. The structural disorder can also lead to the emergence of non-equilibrium phenomena such as glassy dynamics, where the material appears to be stuck in a frozen state. Understanding the local structure of disordered systems is key to explaining their properties and behavior. Local structural motifs and correlations can provide insights into the system’s overall behavior and help establish predictive models. Tailoring the local structure of disordered systems can allow for the creation of materials with specific functionalities such as controlling electronic conductivity, catalytic activity, and thermal insulation. X-ray absorption spectroscopy (XAS) is a powerful technique for probing the local structure and electronic properties of materials, particularly at the atomic level. In this context, we have carried out XAS measurements on BiS2-based systems, high-entropy alloys (HEAs) and highly disordered systems such as TMs oxides and nanoparticles to investigate their local structure and electronic properties. In summary, we have found that the functional properties in BiS2-materials are driven by both the Bi-S axial bond distance and the in-plane BiS2 distortion acting on the charge transfer mechanism and the charge mobility, respectively. The high susceptibility of the local structure to external stimuli such as doping, chemical pressure and temperature makes these materials of particular interest for tailoring the local structure for the desired properties. The local structure also affects the metal-insulator transition in the TMO Ba3Nb5-xTixO15 driven by the local octahedral distortion and configuration disorder rather than temperature. Further studies on thermal conductivity and Seebeck coefficient could optimize the thermoelectric properties of these materials by manipulating local disorder through substitution. Then, we have found that the Zr-Zr network in the TrZr2 become more rigid increasing the configurational entropy suggesting the presence of nanoscale texturing and possible local order like granular materials.