Vibrational and Structural Properties of the RFe4Sb12 (R = Na, K, Ca, Sr, Ba) Filled Skutterudites

• Ano de defesa: 2022
• Autor(a) principal: Abrantes, Juliana Gonçalves de
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: escuteruditas preenchidas; espectroscopia de absorção de raios-x; filled skutterudites; materiais termoelétricos; thermoelectric materials; x-ray absorption spectroscopy
• Link de acesso: https://www.teses.usp.br/teses/disponiveis/43/43134/tde-09082022-112425/

Skutterudites form a large family of intermetallic solid state functional materials presenting a high potential for thermoelectric applications. They are known to present a wide range of physical properties related to quantum many-body effects, such as superconductivity and magnetism, and in some instances strong electronic correlations giving rise to unexpected physical phenomena, that are sensitive to tuning parameters such as pressure, magnetic fields and chemical substitution. The resulting phase diagrams are rich and diverse. Accordingly, this type of material is in the spotlight of studies in many fields of research, given the vast number of phenomenological possibilities that remain unexplored. In fact, our group has a longstanding interest in the properties of skutterudite materials and in particular in the subject of this dissertation: the RFe4Sb12 (R = Na, K, Ca, Sr and Ba) filled skutterudites. This particular series is an example of one of the most challenging problems in condensed matter physics, which is the description of magnetic properties of the itinerant magnets. In this type of magnetism, the magnetic moment per magnetic atom site (in this case, the Fe site) is much smaller than expected for localized spins, suggesting their delocalization. Thence, itinerant magnets are also called small moment magnets. We addressed their electronic properties in order to investigate the nature of their magnetism and XANES (X-ray Absorption Near Edge Structure) experiments were performed, probing the local electronic structure of the FeSb3 coordination. It was noteworthy to find that this local structure could be connected to their magnetic behaviour, despite the character of their magnetism being itinerant. Nevertheless, what seems the main subject triggering most of the skutterudite oriented research is indeed their large potential for thermoelectric applications. Hence, in this dissertation we turn our attention to other properties of the RFe4Sb12 (R = Na, K, Ca, Sr and Ba) series, namely their vibrational and structural properties, all of which intimately relate to their thermoelectric properties. This way, we are able to present an integral picture of this series of skutterudites, so as to unravel the effect of the R filler atom substitution among them. In a first approximation, the good thermoelectric properties of R filled skutterudites are assigned to specific phonon modes inserted in the system through the presence of the R filler cations, that are believed to be weakly coupled to the rest of the structure. In this dissertation, we show that this scenario is not outright, and does not comprise all the R filled materials. This is exposed in the first set of results, that examined the vibrational dynamics of the materials by means of X-Ray Absorption Spectroscopy (XAS) experiments. Additionally, a systematic structural analysis of the materials is conducted via X-Ray Diffraction (XRD) as function of pressure, where the elastic properties of the materials are probed. These lead to theoretical calculations about the bonding properties, aiming to unwind some unexpected features of our experimental fidings. Altogether, with the aid of EXAFS and XRD experiments, we hope to deliver a comprehensive study of the thermoelectric RFe4Sb12 (R = Na, K, Ca, Sr and Ba) filled skutterudites, providing a complete overview of their vibrational, elastic and bonding properties.