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

Titolo della tesi: Study of the local structure and electronic properties of AgBiSe2-based thermoelectric materials

This thesis presents a detailed investigation of the local atomic structure, electronic properties, and vibrational dynamics in Ag-based high-entropy thermoelectric materials, specifically the AgBiSe2−2xTexSx and AgBi1−xSbxSe0.8S0.6Te0.6 systems, utilizing advanced spectroscopic techniques such as temperature- dependent X-ray Absorption Fine Structure (XAFS) and X-ray Photoelectron Spectroscopy (XPS). Systematic substitution studies demonstrate the critical role of local structural modifications and electronic structure changes in optimizing thermoelectric performance. For AgBiSe2−2xTexSx, XAFS analyses reveal significant local distortions and bond length distributions that progressively narrow upon substitution, stabilizing a cubic phase characterized by enhanced disorder yet reduced thermal conductivity. Concurrently, XPS studies uncover anomalous chemical potential shifts, elec- tronic inhomogeneities, and increasing sulfur vacancies, emphasizing the complexity of balancing structural and electronic disorder to maximize thermoelectric efficiency. Further substitution at the Bi site with Sb in AgBi1−xSbxSe0.8S0.6Te0.6 introduces additional phonon scattering centers, systematically reducing local distortions and increasing bond rigidity, as evi- denced by XAFS Einstein model fits. XPS results indicate dual valence states for Sb and Bi, suggesting intricate electronic interactions and local structural inhomogeneities, which significantly influence transport properties. Finally, attempts at carrier doping with Ag1−xNbxBiSe2−2ySyTey have been carried out in both the cubic and trigonal phases. XPS results indicate an enhanced susceptibility of the trigonal pahse samples to Nb introduction, with cubic phase samples showing less aprreciable changes. These re- sults are corroborated by temperature-dependent XAFS measurements, which evidence a tendency to reduced local distortion in the high-concentration Nb trigonal phase systems, while the rock-salt phase does not show appreciable changes, essentially mirroring transport measurement results ob- tained on the same systems. Overall, this thesis provides critical insights into the interplay between local structural disorder, electronic structure, and vibrational dynamics, offering a comprehensive understanding of the mech- anisms underlying enhanced thermoelectric performance in high-entropy Ag-based materials. These findings guide the strategic design of advanced thermoelectric materials, leveraging controlled dis- order and electronic tuning to achieve optimized thermoelectric efficiency.