Structural and physical properties of rare-earth clathrates

Abstract

Clathrates that contain rare-earth elements as guest atoms have been of active interest since the discovery of intermetallic clathrates. A large body of work focussed on thermoelectric properties of Eu-containing clathrates. The very low lattice thermal conductivities that are reached in Eu-containing type-I clathrates are generally attributed to the pronounced rattling of Eu in oversized host cages and to the occurrence of split sites in the larger of the two cages of the structure. The potential of Eu-containing clathrates for magnetic refrigeration has been recognized more recently. Here, key features are the large magnetic moment of Eu, together with the second order character of the paramagnetic to ferromagnetic phase transition. The incorporation of other magnetic rare-earth elements into the clathrate cages has long remained elusive. Only very recently the successful synthesis of a cerium containing type-I clathrate was reported. Interestingly, a sizable enhancement of the thermopower is observed and attributed to a rattling enhanced Kondo interaction. This discovery may trigger a wealth of future investigations.