Związki klatkowe; Układy Kondo; Własnościach termodynamicznych i termoelektrycznych; QCP
Katowice : Uniwersytet Śląski
Cerium-based materials of Ce3M4Sn13 and Ce3M3Sb4 type structure, where M is d-electron metal, are so-called Kondo systems (they contain magnetic atoms) with interesting thermodynamic and thermoelectric properties. In case of Ce3M4Sn13, where M = Ru, Rh Co, there will be possible to observe a quantum critical point (QCP), whereas Ce3M3Sb4 materials have attractive thermoelectric properties. The aim of presented studies was to conduct a complex investigation of electronic structure as well as thermodynamic, magnetic and thermoelectric properties of these compounds. The dissertation was prepared in form of a guide, which consist of 5 scientific articles, related to the subject of dissertation. The investigation of Ce3M4Sn13 compounds was mainly focused on low-temperature basic research, while in case of Ce3M3Sb4, special attention was paid to its thermoelectric properties. The polycrystalline samples were obtained by arc-melting of substrates (pure chemical elements) and high-temperature annealing in order to enforce formation of homogeneous crystalline structure. The Ce3Co4Sn13 sample was also prepared in form of highly-grained ingot in order to investigate the granulation effect on thermoelectric properties. The temperature studies of heat capacity, electrical resistivity, thermal conductivity and thermopower were conducted with PPMS platform. Magnetic measurements were carried out on SQUID magnetometer. Chemical composition, structural and microstructural properties was in turn examined using XRD, XPS, SEM and TOF-SIMS techniques.
The studies of Ce3M4Sn13 have not revealed presence of quantum critical point. Any of investigated materials do not exhibit properties typical for QCP, i.e. long-range magnetic order and non-Fermi liquid behavior. In case of Ce3-xLaxRh4Sn13 series there was observed transition from Kondo lattice to single-impurity Kondo state. Numerical simulation of charge density within the unit cell of Ce3M4Sn13 exhibited similar bonding in Ce3Co4Sn13 and Ce3Rh4Sn13 while quite different in Ce3Ru4Sn13. The thermoelectric measurements of Ce3Co4Sn13 have shown very low thermopower and figure of merit ZT, what excludes the material as candidate for thermoelectric material. It was shown, that Ce3Cu3Sb4 is instead a good base material for further chemical and structural modification in terms of improvement of thermoelectric properties. The substitution of Cu with Ni atoms led to considerable ZT enhancement. There was obtained ZT ≈ 0.3 at 100°C in Ce3Cu2.75Ni0.25Sb4, which is comparable value to that observed in popular thermoelectrics materials.