Dynamical Properties of Skutterudite-Compounds

Although the Seebeck- and Peltier effects are known for more than a century, applications are still rare due to the low efficiency of current thermoelectric materials. A recent breakthrough based on the Phonon Lattice and Electron Crystal concept led to novel thermoelectric materials which envisage applications such as (i) heat pumps (cooling) operating on electric current or (ii) generators of electricity from thermal waste heat sources. There is an increasing demand in modern refrigeration (particularly spot cooling of electronic equipment) as well as in electric power generation (in hybrid automobile applications etc.). Major advantages over conservative competitive systems are: high reliability, silent motionless operation, saving waste energies and being environment-friendly. In order to improve the figure of merit of thermoelectric materials, one of the key parameters to be minimized is the lattice thermal conductivity. It is thus essential to understand the lattice dynamics, especially the scattering mechanism of low energy phonons. Binary and ternary Skutterudites are a class of thermoelectric materials which currently are topic of intense research. Measurements of the specific heat indicate a huge change in the phonon spectra when the interstitital positions of the Skutterudite host lattice are occupied, however measurements of the phonon dispersions have not been performed. Therefore the lattice dynamics of Skutterudite compounds will be studied in this project. The most important technique to be used is neutron spectroscopy both on single crystals and powder specimens. In addition inelastic X- ray scattering experiments are envisaged, if available sample size or other reasons such as high absorption corroborate the use of neutrons. Because in many applications the thermoelectrical materials have to be used as thin films, it is necessary to compare the diffraction pattern and the phonon spectra obtained on bulk polycrystalline material with those on thin films. We propose such investigations on technologically relevant materials showing a high figure of merit. If magnetic ions are placed at the open cage positions of the lattice, the scattering processes are strongly influenced and large variations and strong temperature dependencies in the thermopower and in the thermal conductivity appear. These effects are due to the coupling of the magnetic, electronic and lattice degrees of freedom resulting for instance in heavy fermion and intermediate valence behaviour or orbital order. The excitation spectrum and its dispersion are fingerprints of these basic interactions and will be determined by inelastic scattering experiments.

Although the Seebeck- and Peltier effects are known for more than a century, applications are still rare due to the low efficiency of current thermoelectric materials. A recent breakthrough based on the Phonon Lattice and Electron Crystal concept led to novel thermoelectric materials which envisage applications such as (i) heat pumps (cooling) operating on electric current or (ii) generators of electricity from thermal waste heat sources. There is an increasing demand in modern refrigeration (particularly spot cooling of electronic equipment) as well as in electric power generation (in hybrid automobile applications etc.). Major advantages over conservative competitive systems are: high reliability, silent motionless operation, saving waste energies and being environment-friendly. In order to improve the figure of merit of thermoelectric materials, one of the key parameters to be minimized is the lattice thermal conductivity. It is thus essential to understand the lattice dynamics, especially the scattering mechanism of low energy phonons. Binary and ternary Skutterudites are a class of thermoelectric materials which currently are topic of intense research. Measurements of the specific heat indicate a huge change in the phonon spectra when the interstitital positions of the Skutterudite host lattice are occupied, however measurements of the phonon dispersions have not been performed. Therefore the lattice dynamics of Skutterudite compounds will be studied in this project. The most important technique to be used is neutron spectroscopy both on single crystals and powder specimens. In addition inelastic X- ray scattering experiments are envisaged, if available sample size or other reasons such as high absorption corroborate the use of neutrons. Because in many applications the thermoelectrical materials have to be used as thin films, it is necessary to compare the diffraction pattern and the phonon spectra obtained on bulk polycrystalline material with those on thin films. We propose such investigations on technologically relevant materials showing a high figure of merit. If magnetic ions are placed at the open cage positions of the lattice, the scattering processes are strongly influenced and large variations and strong temperature dependencies in the thermopower and in the thermal conductivity appear. These effects are due to the coupling of the magnetic, electronic and lattice degrees of freedom resulting for instance in heavy fermion and intermediate valence behaviour or orbital order. The excitation spectrum and its dispersion are fingerprints of these basic interactions and will be determined by inelastic scattering experiments.