Strongly correlated cage compounds

Strongly correlated cage compounds are an emergent class of materials conceived to unify, within a single material, the specific properties of strongly correlated electron systems (SCES) and cage compounds. SCES are characterized by strong Coulomb interactions leading to extraordinary phenomena like high-temperature superconductivity, colossal magnetoresistance, or heavy-fermion behaviour. This latter is due to a Kondo-type interaction between localized magnetic moments (of an incompletely filled f shell of a rare earth element) and conduction electrons which results, in particular for systems with low charge carrier concentrations, in giant values of thermopower. Cage compounds are guest/host systems where guest atoms are situated in oversized cages made up of the host species. The strong scattering of the heat-carrying phonons from the oscillating ("rattling") guest atoms leads to low and "glass-like" phonon thermal conductivities. The electrical conductivity, however, is much less affected by the rattling because the conduction electrons are confined to the framework, i.e., the electrical conductivity remains "crystal-like". It is these specific properties - the large thermopower of SCES and the large electrical to thermal conductivity ratio of cage compounds - that shall be joined in representatives of the new class of strongly correlated cage compounds with the aim of "designing" thermoelectric materials with improved efficiencies, particularly below room temperatures. In the proposed project we plan to follow two main routes to reach this goal. The first route deals with clathrates - compounds with a well-defined cage structure. Strong correlations shall be introduced into these by incorporating adequate rare earth elements as guests. This is a challenging project for no group worldwide has so far accomplished this goal. Our preliminary experiments on Yb containing clathrates look, however, very promising. The second route concerns other non-clathrate cage compounds. In order to find entirely new representatives we shall focus on the investigation of (mostly ternary) phase diagrams of an adequate rare earth element, a transition metal, and a third or fourth group element, - in particular in regions with low rare earth element concentration. In addition, we plan to search in crystallographic data bases for known yet largely unexplored cage compounds ("structural data mining``). The synthesis of pure poly- and single-crystalline specimens shall be performed in intimate interaction with structural, analytical, and physical characterizations which give important feedback to improve the synthesis conditions and thus the sample quality. Then, comprehensive measurements of various physical properties in wide parameter ranges shall be performed and interpreted, if appropriate in collaboration with theoreticians. Insights from these investigations are likely to provide clues on how to modify a given material in order to finally make strongly correlated cage compounds with excellent thermoelectric performance reality.

The FWF project P19458-N16 has made important contributions in establishing strongly correlated materials as attractive new science topic, and as substances with high application potential. This new class of materials unifies, within a single material, the specific properties of strongly correlated electron systems (SCES) and cage compounds. SCES are characterized by strong Coulomb interactions leading to extraordinary phenomena like high- temperature superconductivity, colossal magnetoresistance, or heavy-fermion behaviour. This latter may result in giant values of the thermopower. Cage compounds are guest/host systems where guest atoms are situated in oversized cages made up of the host species. The very low thermal conductivities of these materials paired with good electrical conductivities - are generally attributed to the oscillations of the guest atoms in the cages. The combination of these specific properties is just what is needed to obtain thermoelectric materials with high efficiency. In simple materials, these properties are typically mutually exclusive. In this project we have, for the first time, succeeded in incorporating rare-earth elements into clathrates - the archetypal cage compounds - that lead to strong correlation effects. We expect this discovery to have major impact on the field. A number of open issues in conventional clathrates were dealt with in great depth. For instance, the mechanism of reduced thermal conductivity was shown to be a phononic low- pass filter effect. Unprecedented was the discovery of a new synthesis technique for clathrates, by which thin films can be deposited in a very cost-effective way. Also clathrate-like compounds were studied. New representatives were found and fascinating properties observed. The cage structure appears to be prone to low-temperature magnetic phase transitions which, by application of a magnetic field, can be fully suppressed to the absolute zero in temperature. In one of our materials we observed quantum critical behaviour that was completely unexpected and calls for new theoretical approaches. We dealt also with the aspect of nanostructuring. Both polycrystals with reduced grain size and nanowires were synthesized. Many of our new findings and insights lead to new project that we are working on now.