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
