Skutterudites are built up from binary compounds with the chemical composition CoAs₃. Binary CoAs₃ was discovered as a mineral on the farm Skutterud (Norway) in 1772 and served as the basis for the precious blue pigment used in luxury goods like fine china. CoAs₃ is a diamagnetic semiconductor characterised by a large void in its crystal structure. Co can be exchanged by Rh or Ir, while As can be substituted by P and Sb without changing the crystalline unit cell. The cavities of skutterudites can be filled up by electropositive elements like monovalent earth alkali (e.g., Na), divalent alkali (like Ba) or trivalent rare earth elements (La). Since these electropositive elements are just loosely bound, heat carrying phonons are intensely scattered on the resulting „rattling modes“. As a consequence, lattice thermal conductivity is dramatically reduced (by about one order of magnitude) driving a similar enhancement of the thermoelectric figure of merit.

Our research activities in this field are focused on:

• The study of ground state properties, which are primarily determined by the filler elements, particularly by rare earth ions. In the context of non-Kramers ions (such as Pr) and crystal electric field splitting in cubic symmetry, exciting physical features such as conventional or unconventional superconductivity, magnetic, quadrupolar or octupolar ordering, heavy fermion and non-Fermi liquid properties have already been found and more of such phenomena are still to be discovered.
• Optimisation of the electrical resistivity, thermopower and thermal conductivity to bring the figure of merit Z = (σ/κ)S² towards a technically useful order of magnitude. This can be realised by fine-tuning the charge carrier concentration, like substituting Co by Fe (one hole is created) or Ni (one electron is added) or by a change of the concentration of the electropositive elements in the cage.

Collaborations: Uni Wien, CNRS Thias (Paris), MPI CPfS Dresden.

Contact: E. Bauer, S. Bühler-Paschen, H. Michor