Low-Energy Phonon Dynamics in Ordered-Disordered Systems: from Correlated Electron Materials to Perspectives for Thermoelectric Applications

The global energy demand continues to rise rapidly, driven by population growth, industrialization, and urbanization. Since the Industrial Revolution, fossil fuels have been the predominant energy source enabling electricity generation, trans-portation, and industrial processes. However, their extensive use poses critical environmental challenges such as pollution, atmospheric degradation, global warming, and ozone depletion. Addressing these issues requires urgent global efforts to transition toward clean, renewable, and sustainable energy technolo-gies. In this context, the exploration of novel materials capable of efficient energy con-version and transport has gained significant attention. Complex ordered-disordered systems have emerged as promising candidates for next-generation green energy applications due to their unique charge and phonon transport phe-nomena. Yet, the fundamental origins of these strongly correlated behaviours remain insufficiently understood, especially the roles played by low-frequency phonons, which are central to lattice dynamics and charge transport processes. This thesis aims to deepen the fundamental understanding of phonon localiza-tion and softening mechanisms, which are pivotal in controlling charge and heat transport behaviours in advanced functional materials. Phonon localization, of-ten arising from lattice disorder and anharmonicity, suppresses the propagation of heat-carrying lattice vibrations, significantly influencing thermal conductivity. Phonon softening, marked by decreased phonon frequencies, indicates lattice in-stabilities driven by electron-phonon coupling, magnetic interactions, or struc-tural phase transitions. These vibrational phenomena profoundly affect the in-terplay between lattice dynamics and electronic states, modulating phenomena such as charge density wave (CDW) formation and transport properties in quan-tum materials. Charge transport is strongly impacted by phonon softening and localization through enhanced electron-phonon scattering and altered carrier mobility, which can induce unconventional electronic phases. For example, in transition metal dichalcogenides (TMDs), CDWs emerge through the condensa-tion of soft phonon modes, reflecting a delicate coupling between lattice vibra-tions and electronic charge modulations. Similarly, metal halide perovskites (MHPs), a class of materials characterized by phonon glass electron crystal (PGEC) behaviour, manifest crystalline electronic conduction coexisting with glass-like phonon transport, the latter governing by phonon localization and mode softening arising from strong lattice anharmonicity and dynamic disorder. To investigate these intricate phenomena, this work combines advanced experi-mental and theoretical techniques: temperature- and polarization-dependent Raman spectroscopy, density functional theory (DFT) vibrational calculations, low-temperature specific heat, and low-temperature thermal conductivity meas-urements. These approaches enable a direct connection between microscopic phonon dynamics and macroscopic charge and heat transport properties. The thesis is organized into two main sections reflecting different material sys-tems and their phonon-charge transport behaviour. The first part explores CDWs in the TMD family, focusing on a 1T-TaSe₂ sample. Through detailed Raman spectroscopy and complementary DFT analyses, this study elucidates the lattice dynamics and atomic displacements driving CDW transitions, offering insights into the coupling of electronic correlations and lat-tice instabilities. This work was undertaken in collaboration with researchers at the University of Bath and Politecnico di Milano, contributing to the publica-tion titled “Identification of Soft-Modes Across the Incommensurate-to-Commensurate Charge Density Waves Transition in 1T-TaSe₂” (Physical Review B). The second part of this thesis focuses on thermoelectric materials (TMs), which convert waste heat into electricity without moving parts or harmful by-products. Improving thermoelectric efficiency requires a combination of both high electri-cal conductivity and ultra-low lattice thermal conductivity, which are hallmarks of PGEC materials. My work centres on investigate MHPs, whose unique struc-tural features – such as loosely bonded atoms, bonding hierarchies, and elas-tically soft lattices – effectively suppress phonon transport while preserving crys-talline pathways for charge conduction. Through low-temperature specific heat and thermal conductivity measurements, complemented by temperature depend-ent Raman spectroscopy and DFT calculations, this research provides direct in-sight into how these structural and chemical traits give rise to phonon-glass be-haviour and influence thermal transport. This integrated methodology establishes a clear connection between microscopic lattice vibrations and macroscopic transport properties, highlighting the pivotal role of low-frequency phonons in mediating lattice instabilities and heat conduc-tion. This work further uncovers the microscopic origins of phonon softening and lo-calization, drive by lattice anharmonicity and dynamic disorder. To disentangle the interplay of structural and chemical factors, perovskites of different dimen-sionalities and chemical compositions have been systematically investigated. By exploring systems that span from three-dimensional to zero-dimensional struc-tures, this thesis systematically addresses how lattice softness, local bonding in-stabilities, and stereo-chemically active lone pairs contribute to the emergence of glass-like phonon behaviour in otherwise crystalline materials. Ultimately, the findings demonstrate that phonon softening and localization in MHPs emerge from the interplay of anharmonic bonding interactions and the in-herent softness of the perovskite network. By bridging atomic-scale vibrational instabilities with the suppression of thermal conductivity, this thesis advances the microscopic understanding of transport phenomena in complex materials and provides guidance for the design of next-generation TMs and energy-related functional compounds. This second part was undertaken in collaboration with Institute of Low Tempera-ture and Structural Research in Wroclaw (Poland), with great support of Dr. Ing. Daria Szewczyk, East China Normal University, Shanghai (China), with valuable contributions of Prof. Chen Jiangang and his team. The work was conducted within the framework of PRIN PNRR project “StopHEAT: Glass-like phonon transport in eco-friendly perovskites for thermoelectric energy generation” under the supervision of Prof. Giovanna D’Angelo.