Tailoring Fe/Ni-based skutterudites for thermoelectric applications: from the synthesis and optimization of bulk materials to the fabrication of thin films

Thermoelectric materials enable the direct conversion of heat into electricity and vice versa. For this reason, they are part of a promising strategy for improving energy efficiency through the recovery of waste heat. Among the different thermoelectric materials, filled skutterudites have attracted considerable interest due to their structural flexibility, tunable electronic properties, and the possibility of reducing lattice thermal conductivity through phonon scattering mechanisms: this effect can be enhanced by varying the filler atoms, introducing disorder on the transition metal site, or partially substituting atoms of the Sb12 framework. This doctoral work aims to investigate Fe/Ni-based filled skutterudites belonging to the family REy(FexNi1-x)4Sb12-δSnδ (RE = rare earth). In these compounds, the electronic structure can be tuned through compositional modifications, including variation of the Fe/Ni ratio, changes in the rare-earth filling fraction, and partial substitution of Sb with Sn. These parameters modify the valence electron count and progressively shift the Fermi level, enabling continuous tuning of carrier concentration and transport regime. At the same time, filler atoms and Sn substitution introduce mass fluctuations and localized vibrational modes that enhance phonon scattering and contribute to the reduction of lattice thermal conductivity. Within each system, the Fe/Ni ratio was systematically varied over a wide compositional range in order to explore the evolution of transport properties. The research first focuses on the synthesis and optimization of bulk skutterudites prepared through melting-quenching-annealing technique. Structural, thermal, and temperature-dependent transport measurements were performed to investigate the evolution of electrical conductivity, Seebeck coefficient, and thermal transport as a function of composition. Through compositional optimization, two bulk systems were found to reach thermoelectric ZT close to 1 at high temperature, demonstrating the effectiveness of the adopted tuning strategy in balancing electronic transport and phonon scattering. In parallel, selected compositions were deposited as thin films by pulsed laser deposition; their study provides a platform to explore thermoelectric properties under controlled growth conditions and reduced dimensionality. Thin film studies were carried out during a four-month research stay at the Shibaura Institute of Technology in Tokyo, Japan. Finally, X-ray absorption spectroscopy was employed to probe the local electronic structure and atomic environment of both filler atoms and transition metals. These measurements were carried out during two beamtime experiments at the European Synchrotron Radiation Facility in Grenoble, at the LISA Beamline BM08, enabling detailed investigation of the local bonding environment and electronic configuration. By combining transport measurements with spectroscopic analysis, this part of the study establishes correlations between compositional tuning, local electronic structure, lattice dynamics, and thermoelectric performance in Fe/Ni-based filled skutterudites.