The ability of various materials to convert heat into electricity is known for long time and dates back to P. Seebeck (1822).
The efficiency of such a conversion process depends on the so called figure of merit Z and is determined by the Seebeck coefficient, the thermal conductivity and by the electrical resistivity of a certain material.
In order to develop materials, promising with respect to a figure of merit of the order of ZT > 1, the class of skutterudites is explored in detail in our research team.
This group of compounds of the form RETM4X12 (RE ... rare earth, TM ... transition metal elements like Fe, Co, Rh, Ru and X ... Sb, P, ...) crystallise in a cubic crystal and form a large cage which accommodates the electropositive element (compare figure 1).
Depending on the particular electropositive element, exciting low temperature properties are found for these skutterudites. Among them are superconductivity, heavy fermion behaviour, non-Fermi-liquid features, magnetic ordering, metal to insulator transition or hopping conductivity.
The magnitude of the Seebeck coefficient, which can be huge - of the order of several hundred muV/K - is related to the actual carrier concentration. Thus, a substitution of the various elements allows to finely tune the carrier number and a crossover from electron - to hole dominated transport may occur.
The loosely bound electropositive element in the cage interacts strongly with the lattice vibrations and thus can dramatically reduce the thermal conductivity.