Structural study and thermoelectric properties of some group 9 ternary metal chalcogenides

Abstract

The crystal structure and thermoelectric properties of the anion-substituted ternary skutterudites MQ1.5Y1.5 (M = Co, Rh, Ir; Q = Ge, Sn; Y = S, Te) have been investigated. A group theoretical analysis based on powder neutron diffraction data of CoGe1.5Te1.5 is presented, revealing new symmetry elements overlooked in previous studies of similar compounds. The new model obtained was applied in a subsequent neutron diffraction study of the sulphides MGe1.5S1.5 (M = Co, Rh, Ir). A resonant scattering synchrotron experiment has also been performed on the tellurides MQ1.5Te1.5 (M = Co, Rh, Ir; Q = Ge, Sn) in order to assess the extent of anion disorder. The thermoelectric properties of all the compounds under study were measured and put into context with both state-of-the-art and new thermoelectric materials. The synthesis of the fully filled skutterudite LaFe3CoGe6Te6 has also been attempted. The results as well as the theoretical background have been presented in a separate results chapter. An attempted synthesis of the ternary skutterudite RhGe1.5Te1.5 led to the unexpected preparation of the equiatomic RhGeTe phase. Replacement of the transition metal atom by other group 9 elements resulted in the synthesis of the phase CoGeTe. The crystal structure of these materials has been investigated using single-crystal and powder X-ray diffraction. These results have been complemented with a study of both their magnetic and electrical properties. Finally, a thermal conductivity measurement provides an assessment of these materials in terms of their thermoelectric properties up to 350 K. During the course of a study of the Co-Sn-S ternary phase diagram, the ternary phase Co3Sn2S2 was synthesized, instead of the sought CoSn1.5S1.5 phase. It had been reported that such compound presented a break in the resistivity vs. temperature plot around 150 K and was ascribed to a likely phase transition, possibly magnetic. As a result of that, a powder neutron diffraction experiment was undertaken in order to shed some light on the origin of such anomaly. Moreover, transport property and magnetic measurements were carried out to obtain a further insight into the electronic nature of Co3Sn2S2. The results obtained support those of the diffraction experiment and form the last results chapter within this thesis.